Single packet reflective polarizer with thickness profile tailored for low color at oblique angles

ABSTRACT

Multilayer optical film reflective polarizers previously considered to have excessive off-axis color can provide adequate performance in an LC display in an “on-glass” configuration, laminated to a back absorbing polarizer of the display, without any light diffusing layer or air gap in such laminate. The reflective polarizer is a tentered-one-packet (TOP) multilayer film, having only one packet of microlayers, and oriented using a standard tenter such that birefringent microlayers in the film are biaxially birefringent. The thickness profile of optical repeat units (ORUs) in the microlayer packet is tailored to avoid excessive perceived color at normal and oblique angles. Color at high oblique angles in the white state of the display is reduced by positioning thicker ORUs closer to the absorbing polarizer, and by ensuring that, with regard to a boxcar average of the ORU thickness profile, the average slope from an ORU(600) to an ORU(645) does not exceed 1.8 times the average slope from an ORU(450) to the ORU(600).

FIELD OF THE INVENTION

This invention relates generally to multilayer optical film reflectivepolarizing films, with particular application to such films that haveonly one packet or stack of alternating polymer microlayers, some beingbiaxially birefringent, and laminates in which such a reflectivepolarizer is bonded to an absorbing polarizer for use in a display. Theinvention also relates to associated articles, systems, and methods.

BACKGROUND

Reflective polarizers are commonly used to enhance the brightness ofliquid crystal (LC) displays and display systems. The LC display systemtypically includes an LC panel, behind which is an illumination assemblyor backlight positioned to provide light to the LC panel. Brightnessenhancement is provided by the reflective polarizer as the result of alight recycling process: light that cannot (because of its polarizationstate) contribute to the display output is reflected by the reflectivepolarizer back into the backlight, where some of the light isre-reflected towards the reflective polarizer in a differentpolarization state that can contribute to the display output and thatpasses through the reflective polarizer toward the user or viewer.

The LC panel includes a layer of liquid crystal material disposedbetween glass panel plates. Furthermore, the LC panel is sandwichedbetween two absorbing polarizer films: a front absorbing polarizer,attached to the front glass plate of the LC panel, and a back absorbingpolarizer, attached to the back glass plate. The brightness-enhancingreflective polarizer is placed somewhere behind the LC panel, and behindthe back absorbing polarizer.

In practice, design details of the reflective polarizer have an impacton exactly where the reflective polarizer can be placed in the displaysystem to provide optimal, or at least acceptable, optical performance.Some types of reflective polarizers can be laminated directly to theexposed rear surface of the back absorbing polarizer. Those of ordinaryskill in the art consider it necessary for these types of reflectivepolarizers to have a very low perceived color for the pass state ofpolarization both at normal incidence (light propagating along theoptical axis of the display system) and at highly oblique incidence.Since the reflective polarizer is attached to the back absorbingpolarizer, and the back absorbing polarizer is in turn commonly attachedto the back glass plate of the LC panel, this is referred to as an“on-glass” configuration of the reflective polarizer. One reflectivepolarizer currently used in the on-glass configuration is aparabolically-stretched reflective polarizer, discussed further below.Another reflective polarizer used in the on-glass configuration is amulti-packet reflective polarizer, also discussed below.

Other types of reflective polarizers, now considered by those ofordinary skill in the art to have excessive perceived color for the passstate of polarization for obliquely incident light, are not laminated tothe back absorbing polarizer of the display because the (undesirable)color associated with the reflective polarizer would be visible to theuser through the absorbing polarizer and through the LC display.Instead, these latter types of reflective polarizers—multilayer opticalfilm reflective polarizers of alternating polymer layers in which thereis only one packet of microlayers, the microlayer packet having athickness gradient or profile to provide broadband reflection, themultilayer optical film having been oriented using a standard tentersuch that birefringent layers of the film are biaxially birefringent,such films referred to herein as Tentered-One-Packet (“TOP”) films orTOP reflective polarizers—are used in the display system as astand-alone film, separated from the back absorbing polarizer by atleast one air gap, and attached to a light diffusing film or layer thatis disposed between the reflective polarizer and the back absorbingpolarizer. The light diffusing layer has a significant haze value so asto effectively combine light rays that pass through the reflectivepolarizer in different directions, to reduce or eliminate the colorassociated with the TOP reflective polarizer from the standpoint of theuser or viewer.

U.S. Pat. No. 7,791,687 (Weber et al.) appears to go against thisprevailing opinion by disclosing embodiments in which a display panelhas on one side thereof the combination of a first absorbing polarizerand a TOP reflective polarizer, these two polarizers being aligned witheach other, and on the other side of the display panel is a secondabsorbing polarizer that is crossed with (oriented at 90 degreesrelative to) the first absorbing polarizer. However, the '687 Weberpatent refers to special cases in which the first absorbing polarizer isa low contrast absorbing polarizer (see e.g. column 2, lines 1-15, andcolumn 3, lines 22-39). In the examples, the first absorbing polarizerhas a contrast ratio of only about 5 (see e.g. Example 2, where theblock state transmission of the first absorbing polarizer is reported as20%). The '687 Weber patent says that in these cases where the firstabsorbing polarizer is of low contrast, the optical properties of thereflective polarizer become more important for maintaining the contrastof the display (see col. 3, lines 22-39). As demonstrated in theexamples, the '687 Weber patent then assesses the display contrast byevaluating the block state (dark state) performance of the display. Thatis, the patent calculates and compares the spectral transmission throughcrossed polarizer systems, in which a combination TOP reflectivepolarizer/first absorbing polarizer (of low contrast, and aligned withthe TOP polarizer) is crossed with a second absorbing polarizer of highcontrast. These transmission spectra are calculated for various obliquepolar angles θ and an azimuthal angle φ of 45 degrees. The calculatedtransmission through such crossed polarizer systems is representative ofthe dark state of the display, and is thus very low-all of the exampleshave transmissions under 4%, and some are well under 1%, over the entirevisible wavelength region for the angles that were tested. The examplescompare systems in which the TOP reflective polarizer is orienteddifferent ways—some where the thickness profile of the TOP reflectivepolarizer is oriented one way, and some where it is oriented theopposite way—by comparing their calculated transmission spectra. Thisanalysis led the '687 Weber researchers to conclude the thicknessprofile of the TOP reflective polarizer should be oriented such that amajority of the layers having a smaller optical thickness are disposedcloser to the display panel than the layers having a larger opticalthickness. In embodiments where the combination TOP reflectivepolarizer/low contrast absorbing polarizer is disposed behind (ratherthan in front of) the display panel, this means that the thicknessprofile of the TOP polarizer should be oriented so that thinner layersface the front, i.e., towards the user, and the thicker layers face theback, i.e., away from the user and towards the backlight.

BRIEF SUMMARY

In light of current prevailing opinion that TOP reflective polarizersare not suitable for on-glass applications in modern display systemsthat use a high contrast absorbing polarizer both at the front and theback of the LC display panel due to the significant color generated byconventional TOP polarizers in the pass state (white state) of thedisplay system at highly oblique incidence angles, we have revisited thesuitability of TOP reflective polarizers for these applications. Brieflysummarized, we have found it is indeed feasible to use TOP reflectivepolarizers in such display systems in an on-glass configuration, i.e.,laminated to the high contrast absorbing polarizer at the back of thedisplay panel. We have further found that unwanted visible color at highoblique angles in the white state of the display can be substantiallyreduced to acceptable levels by properly orienting the TOP polarizer,and by properly tailoring the layer thickness profile associated withthe thick microlayer end of the microlayer packet. Interestingly, theorientation of the TOP polarizer we have found to be optimal—having thethicker microlayers (more precisely, the thicker optical repeat units(ORUs)) face the front of the display (and the absorbing polarizer), andthe thinner microlayers (more precisely, the thinner ORUs) face the backof the display—is the opposite of the orientation taught by the '687Weber et al. patent.

TOP reflective polarizers, properly designed and oriented, can provideacceptable performance in an LC display, in an on-glass configuration,without the need for any air gap or high haze light diffusing layer.Thus, a laminate made by combining such a TOP reflective polarizer witha high contrast absorbing polarizer, without an air gap and without ahigh haze light diffusing layer or structure (and in some cases withoutany significant light diffusing layer or structure at all) between thereflective polarizer and the absorbing polarizer, can be successfullyused and incorporated into a liquid crystal display or the like. The TOPreflective polarizer in this construction is a multilayer optical filmof alternating polymer layers in which there is only one packet ofmicrolayers, the multilayer optical film having been oriented using astandard tenter such that birefringent layers (including microlayers) ofthe film are biaxially birefringent. The microlayers in the packet, ormore precisely the ORUs in the packet, are provided with a thicknessprofile appropriately tailored to avoid excessive perceived color atnormal and highly oblique angles for the pass state (white state) of thedisplay system. Such TOP multilayer optical film reflective polarizersare discussed further below.

We thus describe herein, inter alia, reflective polarizers having onlyone packet of microlayers that reflects and transmits light by opticalinterference, the packet of microlayers configured to define a firstpass axis (y), a first block axis (x), and a first thickness axis (z)perpendicular to the first pass axis and the first block axis. Thepacket of microlayers may include alternating first and secondmicrolayers, at least the first microlayers being biaxiallybirefringent. Adjacent pairs of the first and second microlayers formoptical repeat units (ORUs) along the packet of microlayers, the ORUsdefining a physical thickness profile having a gradient that provides awide band reflectivity for normally incident light polarized along thefirst block axis. The ORUs have respective resonant wavelengths as afunction of the physical thickness profile and optical geometry. TheORUs include a first ORU and a last ORU at opposite ends of the packet.ORUs proximate the last ORU have an average physical thickness greaterthan that of ORUs proximate the first ORU. An intrinsic-bandwidth basedboxcar average of the physical thickness profile yields an IB-smoothedthickness profile, the IB-smoothed thickness profile being defined ateach of the ORUs. The ORUs include an ORU(450), an ORU(600), and anORU(645). The ORU(450) has a resonant wavelength, for the IB-smoothedthickness profile, of at least 450 nm for an oblique optical geometry inwhich p-polarized light is incident in the x-z plane at a polar angle(θ) of 80 degrees. All of the ORUs disposed on a side of the ORU(450)that includes the first ORU have resonant wavelengths, for theIB-smoothed thickness profile, less than 450 nm for the oblique opticalgeometry. The ORU(600) and ORU(645) are similarly defined, and haveresonant wavelengths of at least 600 nm and 645 nm, respectively, at thesame oblique optical geometry. The physical thickness profile of thepacket is tailored such that the IB-smoothed thickness profile has afirst average slope over a range from ORU(450) to ORU(600), and a secondaverage slope over a range from ORU(600) to ORU(645), and the ratio ofthe second average slope to the first average slope is no more than 1.8.

By satisfying this condition, the TOP reflective polarizer, and thelaminate of which it is a part, can impart an amount of color—to whitelight passing through it at highly oblique angles—that is so small that,for a display incorporating such a polarizer or laminate, the perceivedcolor of the white state of such display at such highly oblique anglesis acceptably close to a neutral white or target white color.

We also describe laminates in which such a reflective polarizer iscombined with an absorbing polarizer. The absorbing polarizer has asecond pass axis and a second block axis, and has a high contrast ratio,e.g., a contrast ratio of at least 1000. The absorbing polarizerattaches to the reflective polarizer with no air gap therebetween, andsuch that the first and second pass axes are substantially aligned. Thereflective polarizer is oriented relative to the absorbing polarizersuch that the last ORU is closer than the first ORU to the absorbingpolarizer.

Related methods, systems, and articles are also discussed.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side or sectional view of a liquid crystal displaysystem;

FIG. 2 is a schematic side or sectional view of a single packetmultilayer optical film configured as a reflective polarizer;

FIG. 3 is a perspective view of a web of optical film;

FIG. 4 is a perspective view of an optical film or laminate in relationto a Cartesian coordinate system;

FIG. 5 is a schematic perspective view of a multilayer optical filmreflective polarizer disposed behind and spaced apart from an absorbingpolarizer, the reflective polarizer provided with a light diffusinglayer to reduce the amount of observed color;

FIG. 6 is a schematic perspective view of a laminate of a multilayeroptical film reflective polarizer and an absorbing polarizer, with nolight diffusing layer;

FIG. 7 is a schematic perspective view of a laminate similar to that ofFIG. 6 but further including a glass layer from a liquid crystal panel,the absorbing polarizer being disposed between the reflective polarizerand the glass layer;

FIG. 8A is a simplified graph for illustrative purposes of a firstphysical thickness profile and a boxcar-smoothed thickness profile forthe microlayer packet of a TOP reflective polarizer, the packetcontaining exactly 15 ORUs, and FIG. 8B is a corresponding simplifiedgraph for the same embodiment but depicting the resonant wavelength forthe boxcar-smoothed thickness profile and for a given optical geometry;

FIG. 9 is a graph of eight different but related physical thicknessprofiles that may be used in a TOP reflective polarizer, the performanceof which was modeled and shown in FIGS. 10A through 17C;

FIG. 10A is a compound graph that plots ORU thickness against ORUnumber, and also plots a resonant wavelength against the ORU number, andFIG. 10B is a graph of the slope of an averaged thickness profile inFIG. 10A as a function of resonant wavelength, and FIG. 10C is a graphof the calculated color of light transmitted through a laminate of a TOPreflective polarizer and a high contrast absorbing polarizer over arange of azimuthal (ϕ) and polar (θ) angles, the TOP reflectivepolarizer having the thickness profile of FIG. 10A;

FIGS. 11A, 12A, 13A, 14A, 15A, 16A, and 17A are compound graphs similarto that of FIG. 10A but for other TOP reflective polarizer embodiments,and FIGS. 11B, 12B, 13B, 14B, 15B, 16B, and 17B are graphs similar tothat of FIG. 10B but for such other TOP polarizer embodiments, and FIGS.11C, 12C, 13C, 14C, 15C, 16C, and 17C are graphs similar to that of FIG.10C but for such other TOP polarizer embodiments;

FIG. 18 is a graph of eight different but related physical thicknessprofiles that may be used in a TOP reflective polarizer, the performanceof which was modeled and shown in FIGS. 19A through 26C;

FIG. 19A is a compound graph that plots ORU thickness against ORUnumber, and also plots a resonant wavelength against the ORU number, andFIG. 19B is a graph of the slope of an averaged thickness profile inFIG. 19A as a function of resonant wavelength, and FIG. 19C is a graphof the calculated color of light transmitted through a laminate of a TOPreflective polarizer and a high contrast absorbing polarizer over arange of azimuthal (ϕ) and polar (θ) angles, the TOP polarizer havingthe thickness profile of FIG. 19A;

FIGS. 20A, 21A, 22A, 23A, 24A, 25A, and 26A are compound graphs similarto that of FIG. 19A but for other TOP reflective polarizer embodiments,and FIGS. 20B, 21B, 22B, 23B, 24B, 25B, and 26B are graphs similar tothat of FIG. 19B but for such other TOP polarizer embodiments, and FIGS.20C, 21C, 22C, 23C, 24C, 25C, and 26C are graphs similar to that of FIG.19C but for such other TOP polarizer embodiments;

FIG. 27 is a graph of three different but related physical thicknessprofiles that may be used in a TOP reflective polarizer, the performanceof which was modeled and shown in FIGS. 28A through 30C;

FIG. 28A is a compound graph that plots ORU thickness against ORUnumber, and also plots a resonant wavelength against the ORU number, andFIG. 28B is a graph of the slope of an averaged thickness profile inFIG. 28A as a function of resonant wavelength, and FIG. 28C is a graphof the calculated color of light transmitted through a laminate of a TOPreflective polarizer and a high contrast absorbing polarizer over arange of azimuthal (ϕ) and polar (θ) angles, the TOP polarizer havingthe thickness profile of FIG. 28A;

FIGS. 29A and 30A are compound graphs similar to that of FIG. 28A butfor other TOP multilayer optical film reflective polarizer embodiments,and FIGS. 29B and 29C, and 30B and 30C are graphs similar to those ofFIGS. 28B and 29C, respectively, but for such other TOP polarizerembodiments, and FIGS. 20C, 21C, 22C, 23C, 24C, 25C, and 26C are graphssimilar to that of FIG. 19C but for such other TOP polarizerembodiments;

FIG. 31 is a graph of a measured physical thickness profile for anexample TOP multilayer optical film reflective polarizer that was madeand tested;

FIG. 32A is a compound graph that plots ORU thickness against ORUnumber, and also plots a resonant wavelength against the ORU number, andFIG. 32B is a graph of the slope of an averaged thickness profile inFIG. 32A as a function of resonant wavelength, and FIG. 32C is a graphof the calculated color of light transmitted through a laminate of a TOPreflective polarizer and a high contrast absorbing polarizer over arange of azimuthal (ϕ) and polar (θ) angles, the TOP polarizer havingthe thickness profile of FIG. 32A;

FIG. 33 is a graph of a measured physical thickness profile for acomparative example (known) TOP reflective polarizer;

FIG. 34A is a compound graph that plots ORU thickness against ORUnumber, and also plots a resonant wavelength against the ORU number, andFIG. 34B is a graph of the slope of an averaged thickness profile inFIG. 34A as a function of resonant wavelength, and FIG. 34C is a graphof the calculated color of light transmitted through a laminate of a TOPreflective polarizer and a high contrast absorbing polarizer over arange of azimuthal (ϕ) and polar (θ) angles, the TOP polarizer havingthe thickness profile of FIG. 34A;

FIG. 35 is a graph of a measured physical thickness profile for anothercomparative example (known) TOP multilayer optical film reflectivepolarizer; and

FIG. 36A is a compound graph that plots ORU thickness against ORUnumber, and also plots a resonant wavelength against the ORU number, andFIG. 36B is a graph of the slope of an averaged thickness profile inFIG. 36A as a function of resonant wavelength, and FIG. 36C is a graphof the calculated color of light transmitted through a laminate of a TOPmultilayer optical film reflective polarizer and a high contrastabsorbing polarizer over a range of azimuthal (ϕ) and polar (θ) angles,the TOP polarizer having the thickness profile of FIG. 36A.

In the figures, like reference numerals designate like elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

As stated above, we have found that TOP (tentered-one-packet) multilayeroptical film reflective polarizers, normally considered to have too muchoff-axis color to be used in an on-glass configuration with a highcontrast absorbing polarizer, can actually provide adequate performancein such a configuration in a liquid crystal display. No air gap or highhaze light diffusing layer (and in some cases no light diffusing layeror structure at all) is needed, and none is typically provided, betweenthe TOP reflective polarizer and the absorbing polarizer, or anywhere ina laminate that comprises these two polarizers. The high contrastabsorbing polarizer is generally located at the back of the LC panel inan LC display, which display may also include a high contrast absorbingpolarizer at the front of the LC panel, as well as front and back glassplates as described above.

The TOP reflective polarizer has only one packet of microlayers, and isoriented using a standard tenter such that birefringent microlayers inthe film are biaxially birefringent as a result of the constrainedstretch of the tenter. Furthermore, the microlayers in the singlepacket—or rather, the optical repeat units (ORUs) defined by themicrolayers—have a suitably tailored thickness profile. The thicknessprofile is tailored so that thicker ORUs in the microlayer packet arecloser than thinner ORUs to the absorbing polarizer. The thicknessprofile is further tailored as described further below to provide thelaminate with a surprisingly low amount of perceived color in lighttransmitted through the TOP reflective polarizer (and through thelaminate of which it is a part) at highly oblique angles. By sotailoring the thickness profile of the microlayer packet, the TOPreflective polarizer, when combined with a high contrast absorbingpolarizer, can provide an acceptable on-glass laminate for use in an LCdisplay. Adequate color performance can be achieved both at normalincidence and oblique incidence, up to a polar angle (θ) of at least 80degrees, and at intermediate azimuthal angles (ϕ) between 0 and 90degrees.

In general, if one is given a multilayer optical film reflectivepolarizer of unspecified design, appropriate placement of thatreflective polarizer in an LC display system is a function of, amongother things, color characteristics of the reflective polarizer,particularly color characteristics at high off-axis (oblique) angles oflight propagation. Good color performance is harder to achieve at highlyoblique angles than at normal incidence. The color characteristics are,in turn, a function of the manner in which the film was fabricated, andthe film's resulting physical and optical features.

For example, it is known to fabricate a reflective polarizer bycoextruding tens, hundreds, or thousands of alternating polymer layersthrough a die, optionally doubling or tripling the number of layers bysplitting and re-stacking the flow stream in a layer multiplier device,cooling the extrudate on a casting wheel, and orienting (stretching) thecast film to reduce the film thickness such that individual polymerlayers form optically thin microlayers, and to induce birefringence inat least some of the microlayers. In the finished multilayer opticalfilm, the microlayers reflect and transmit light by opticalinterference, as a function of refractive index differences betweenadjacent microlayers, optical thicknesses of adjacent pairs ofmicrolayers, and the thickness profile of a stack of such layer pairsalong a thickness direction or axis of the film. To make a reflectivepolarizer, the orientation or stretching is carried out primarily alongone in-plane direction, so that the refractive indices of themicrolayers define a block axis of high reflectivity, a pass axis of lowreflectivity (and high transmission), and a thickness axis perpendicularto the pass and block axes. See for example U.S. Pat. No. 5,882,774(Jonza et al.). FIG. 1 is provided for reference to illustrate variouscomponents, layers, and films that may be included in a typical LCdisplay system 100. The display system 100 includes a display panel 150and an illumination assembly 101 positioned behind the panel 150 toprovide light thereto. The display panel 150 can include any suitabletype of display. In the illustrated embodiment, the display panel 150includes, or is, an LC panel (hereafter referred to as LC panel 150).The LC panel 150 typically includes a layer of liquid crystal (LC) 152disposed between panel plates 154 a, 154 b (collectively, 154). Theplates 154 are often composed of glass and can include electrodestructures and alignment layers on their inner surfaces for controllingthe orientation of the liquid crystals in the LC layer 152. Theseelectrode structures are commonly arranged so as to define LC panelpixels, i.e., areas of the LC layer where the orientation of the liquidcrystals can be controlled independently of adjacent areas. A colorfilter may also be included with one or more of the plates 152 forimposing desired colors such as red, green, and blue on subpixelelements of the LC layer, and thus on the image displayed by the LCpanel 150.

The LC panel 150 is positioned between a front (or upper) absorbingpolarizer 156 and a back (or lower) absorbing polarizer 158. In theillustrated embodiment, the front and back absorbing polarizers 156, 158are located outside the LC panel 150. Often, the absorbing polarizer(156 or 158) is laminated to the outer major surface of its neighboringglass panel plate (154 a or 154 b respectively) with a suitabletransparent adhesive. The absorbing polarizers 156, 158 and the LC panel150 in combination control the transmission of light from a backlight110 through the display system 100 to the viewer. For example, theabsorbing polarizers 156, 158 may be arranged with their pass axes(transmission axes) perpendicular to each other. Selective activation ofdifferent pixels of the LC layer 152, e.g. by a controller 104, resultsin light passing out of the display system 100 at certain desiredlocations, thus forming an image seen by the viewer. The controller 104may include, for example, a computer or a television controller thatreceives and displays television images.

One or more optional layers 157 may be provided proximate the frontabsorbing polarizer 156, for example, to provide mechanical and/orenvironmental protection to the display surface. The layer 157 may forexample include a hardcoat over the front absorbing polarizer 156.

The illumination assembly 101 includes a backlight 110 and one or morelight management films in an arrangement 140 positioned between thebacklight 110 and the LC panel 150. The backlight 110 can be or includeany known backlight of suitable design. For example, light source(s)within the backlight may be positioned such that the backlight is of theedge-lit variety or the direct-lit variety. The light source(s) mayinclude any known light sources, including one or more of: fluorescentbulbs or lamps, including cold cathode fluorescent lamps (CCFLs); andindividual LEDs or arrays of LEDs, typically, LEDs that emit nominallywhite light, whether by a combination of different colored LED die chips(such as RGB), or by a blue or UV LED die illuminating and exciting awhite- or yellow-light-emitting phosphor.

The arrangement 140 of light management films, which may also bereferred to as a light management unit, is positioned between thebacklight 110 and the LC panel 150. The light management films affectthe illumination light propagating from the backlight 110. In some casesthe backlight 110 can be considered to include one, some, or all of thelight management films in the arrangement 140.

The arrangement 140 of light management films may include a diffuser148. The diffuser 148 is used to scatter or diffuse the light receivedfrom the backlight 110. The diffuser 148 may be any suitable diffuserfilm or plate. For example, the diffuser 148 can include any suitablediffusing material or materials. In some embodiments, the diffuser 148may include a polymeric matrix of polymethyl methacrylate (PMMA) with avariety of dispersed phases that include glass, polystyrene beads, andCaCO₃ particles. The diffuser 148 may also be or include 3M™ Scotchcal™Diffuser Film, types 3635-30, 3635-70, and 3635-100, available from 3MCompany, St. Paul, Minn., USA. A diffuser 148 as used in a lightmanagement film arrangement such as arrangement 140 would typically havea relatively high haze, e.g. at least 40%, as measured using a HazeGuard Plus haze meter from BYK-Gardiner, Silver Springs, Md., accordingto a suitable procedure such as that described in ASTM D1003.

The light management unit 140 also includes a reflective polarizer 142.Although in a general sense the reflective polarizer 142 may be of anysuitable design—for example, a multilayer optical film, a diffuselyreflective polarizing film (DRPF) such as a continuous/disperse phasepolarizer, a wire grid reflective polarizer, or a cholesteric reflectivepolarizer—for purposes of the present application we are interested incases where the reflective polarizer is a particular type of multilayeroptical film, as discussed elsewhere herein. For example, the reflectivepolarizer may be a TOP reflective polarizer as described above. Those ofordinary skill in the art have regarded this type of reflectivepolarizer as having so much off-axis color that a high haze diffuser andair gap between the reflective polarizer 142 and the back absorbingpolarizer 158 was considered necessary to keep the overall perceivedcolor of the display system 100 at or reasonably near a neutral whitecolor, in the pass state (white state) of the display system.

In some embodiments, a polarization control layer 144, such as a quarterwave retarding layer, may be provided between the diffuser 148 and thereflective polarizer 142. The polarization control layer 144 may be usedto change the polarization of light that is reflected from thereflective polarizer 142 so that an increased fraction of the recycledlight is transmitted through the reflective polarizer 142.

The arrangement 140 of light management films may also include one ormore brightness enhancing layers. A brightness enhancing layer canredirect off-axis light in a direction closer to the axis of thedisplay. This increases the amount of light propagating on-axis throughthe LC layer 152, thus increasing the brightness of the image seen bythe viewer. One example of a brightness enhancing layer is a prismaticbrightness enhancing layer, which has a number of prismatic ridges thatredirect the illumination light through refraction and reflection. InFIG. 1, a first prismatic brightness enhancing layer 146 a providesoptical gain in one dimension, and a second prismatic brightnessenhancing layer 146 b has prismatic structures oriented orthogonally tothose of layer 146 a, such that the combination of layers 146 a, 146 bincreases the optical gain of the display system 100 in two orthogonaldimensions. In some embodiments, the brightness enhancing layers 146 a,146 b may be positioned between the backlight 110 and the reflectivepolarizer 142.

The different layers in the light management unit 140 may be freestanding relative to each other. Alternatively, two or more of thelayers in the light management unit 140 may be laminated to each other.

Two design aspects of the multilayer optical film reflective polarizerto be used in the LC display system are of particular relevance to thepresent application: the manner in which the extruded film isstretched—which in practical effect determines whether the birefringentmicrolayers are uniaxially birefringent or biaxially birefringent —, andwhether layer multiplier devices are used during fabrication, or whetherthe finished multilayer optical film has more than one distinct stack orpacket of microlayers.

We first discuss the manner of stretching or orienting the extrudedfilm. In a first known technique, a long length or web of polymer filmcontinuously advances through a standard tenter apparatus. In thestandard tenter, the film is held tautly by sets of clips attached toopposite edges of the film, and the clip sets move forward along rails,under the action of a chain drive or the like. In one section of thetenter, straight sections of the rails diverge from each other such thatthe clips stretch the film in the cross-web direction (also called thetransverse direction) as the clips carry the film generally forward inthe down-web direction (also called the longitudinal direction). Thisorients the film primarily in the cross-web direction. The clips in thestandard tenter maintain a constant clip-to-clip spacing and move at aconstant speed throughout the length of the straight rail sections,which prevents the film from relaxing in the down-web direction. Due tothis down-web constraint of the film during orientation, the stretchprovided by such a standard tenter is sometimes referred to as aconstrained stretch. As a consequence of the constraint, layers withinthe film that become birefringent under the conditions of the stretchtypically develop three different refractive indices along the threeprincipal directions (the cross-web or x-direction, the down-web ory-direction, and the thickness or z-direction) of the film. If we denotethe refractive indices of such a layer along the principal x-, y-, andz-directions as nx, ny, and nz, then nx≠ny, and ny≠nz, and nz≠nx. (Tothe extent the material exhibits dispersion, whereby a given refractiveindex n changes somewhat as a function of optical wavelength, therefractive index may be understood to be specified at a particularvisible wavelength such as 550 nm (green) or 632.8 nm (He—Ne laser,red), or the refractive index may be understood to be an average overthe visible wavelength range, e.g. from 400-700 nm.) A material havingthis type of birefringence is said to be biaxially birefringent.

In a reflective polarizer in which birefringent microlayers alternatewith isotropic microlayers, a consequence of the birefringentmicrolayers being biaxially birefringent is that the layer-to-layerrefractive index differences along the y-direction and along thez-direction cannot both be zero. This in turn results in residualreflectivity and (when used in a display) perceived color for light thatpropagates at high oblique angles relative to an optical axisperpendicular to the film, for p-polarized light propagating in areference plane that includes the y-axis (i.e., the pass axis of thepolarizer) and the z-axis, and for highly oblique light propagatingalong other directions.

In a second known technique, the film or web advances through astretching apparatus that has been specially designed to allow the webor film to fully relax in the down-web direction during the orientationprocess. For example, in some embodiments the stretching apparatusutilizes sets of clips that move along parabolically-shaped rails. Seee.g. U.S. Pat. No. 6,949,212 (Merrill et al.). By allowing the film torelax in the down-web direction (as well as in the thickness direction),layers within the film that become birefringent under the conditions ofthe stretch typically develop only two different refractive indicesalong the three principal directions of the film. Stated differently,for such a birefringent layer, the refractive index along thez-direction equals, or substantially equals, the refractive index alongthe y-direction, but those refractive indices differ substantially fromthe refractive index along the x-direction (the direction of stretch).Using the nx, ny, nz notation, ny=nz, but nx≠ny, and nx≠nz. (In somecases ny and nz may not be exactly equal, but their difference is verysmall, as discussed below. Thus, ny≈nz.) A material having this type ofbirefringence is said to be uniaxially birefringent. In a reflectivepolarizer in which birefringent microlayers alternate with isotropicmicrolayers, a consequence of the birefringent microlayers beinguniaxially birefringent is that the layer-to-layer refractive indexdifferences along the y-direction and along the z-direction can both bemade to be zero, or substantially zero, while the refractive indexdifference along the x-direction is nonzero and large in magnitude. Thisresults in little or no significant reflectivity at high oblique angles,and little or no perceived color at such angles when the film is used asa reflective polarizer in a display.

Thus, with regard to off-axis color in a display, a multilayerreflective polarizer whose birefringent microlayers are uniaxiallybirefringent, e.g. made using a parabolic stretching apparatus, has aninherent advantage relative to a polarizer whose birefringentmicrolayers are biaxially birefringent, e.g. made using a conventionaltenter. However, in practice, with all other factors being equal, auniaxially birefringent polarizer is more costly to manufacture than abiaxially birefringent polarizer, at least in part due to substantiallylower yields for the specialized parabolic stretching apparatus comparedto those for the standard tenter.

Optical materials that may be used in the fabrication of the disclosedreflective polarizers can be selected from known materials, preferablytransparent polymer materials whose material properties allow for thecoextrusion of such materials at the same temperature and in a commonfeedblock. In exemplary embodiments, layers of alternating thermoplasticpolymers (ABABAB . . . ) are used, and one of the polymers is selectedto become birefringent, and the other polymer is selected to remainoptically isotropic, under the conditions of stretching. Suitablepolymers may be judiciously selected from, for example, polyethylenenaphthalate (PEN), polyethylene terephthalate (PET), polybutyleneterephthalate (PBT), copolymers thereof, and blends thereof.Additionally, other classes of polymers that exhibit birefringence andmay be useful for this purpose are polystyrenes (including syndiotacticpolystyrene), polyamides (including Nylon6), and liquid crystalpolymers.

Regarding the above discussion relating to uniaxial and biaxialbirefringence, and equalities and inequalities involving nx, ny, and nz,we recognize that exact equality between two refractive indices may bedifficult to achieve or measure, and, from a practical standpoint, smalldifferences may be indistinguishable from exact equality. Therefore, forpurposes of this document, we consider a material to be uniaxiallybirefringent if one pair of its refractive indices are substantially thesame, e.g., if they differ by less than 0.05, while remaining pairs ofits refractive indices are not substantially the same, e.g., if theydiffer by at least 0.05. Likewise, a material is considered to bebiaxially birefringent if each and every pair of its principalrefractive indices are not substantially the same, e.g., if they differby at least 0.05.

Typically, with regard specifically to multilayer optical filmreflective polarizers, a biaxially birefringent layer in such polarizermay for example have refractive indices nx, ny, nz that satisfy therelationships |ny−nz|≥0.05, and |nx−ny|>0.06 or 0.08. In contrast, auniaxially birefringent layer in such polarizer may for example haverefractive indices nx, ny, nz that satisfy the relationships|ny−nz|≥0.05, and |nx−ny|>0.06 or 0.08.

Another design aspect of particular relevance to the present applicationis the number of distinct stacks of microlayers that are present in thefinished multilayer reflective polarizer, which is often related towhether layer multiplier devices were used during fabrication of thefilm. In describing this feature, reference is made to FIG. 2, whichschematically depicts a single packet multilayer optical film configuredas a reflective polarizer 220.

The multilayer optical film or polarizer 220 has two opposed outer majorsurfaces 220 a, 220 b, between which are a plurality of distinct polymerlayers. Polymer materials and film-making equipment that can be used tomake such a film by coextrusion and stretching are known, see e.g. U.S.Pat. No. 5,882,774 (Jonza et al.) and U.S. Pat. No. 6,783,349 (Neavin etal.), and patent application publication US 2011/0102891 (Derks et al.).Adjacent polymer layers have substantially different refractive indicesalong at least one of the principal x, y, or z axes, so that some light(depending upon the direction of propagation and the polarization stateof the light) is reflected at interfaces between the layers. Some of thepolymer layers of the polarizer 220 are sufficiently thing referred toherein as “optically thin”—so that light reflected at a plurality of theinterfaces undergoes constructive or destructive interference in orderto give the multilayer optical film the desired reflective ortransmissive properties. These layers are referred to herein asmicrolayers, and are labeled “A” and “B” in FIG. 2. For reflectivepolarizers designed to reflect visible light, each microlayer generallyhas an optical thickness (i.e, a physical thickness multiplied by itsrefractive index) of less than about 1 micron. Thicker layers, such asskin layers or protective boundary layers (PBLs), as are known in theart, may also be present in the polarizer, as illustrated in FIG. 2 bythe layer 222. Such “optically thick” layers have an optical thicknessof at least 1 micron, and often much greater than 1 micron, and are notconsidered to be microlayers. (Throughout this document, when the term“thickness” is used without the modifier “optical”, the thickness refersto the physical thickness, unless otherwise indicated by the context.)

A coherent grouping of microlayers is referred to herein as a stack orpacket of microlayers, or as a microlayer packet. As shown, thepolarizer 220 contains only one packet 224 of microlayers. The packet224 has a (physical) thickness of T₁, and the polarizer 220 has anoverall thickness of T₂, as shown in the figure. Configuring themultilayer optical film with only one packet 224 of microlayerssimplifies the manufacturing process (provided the number of microlayersdesired is not excessive) and allows for greater control of thethicknesses and thickness profiles of the microlayers, which in turnallows for greater control of the spectral reflectivity and spectraltransmission characteristics of the reflective polarizer. In FIG. 2,pairs of adjacent microlayers form optical repeat units (ORUs), labeledORU1 through ORU6, each ORU having an optical thickness (OT1, OT2, OT6)equal to the sum of the optical thicknesses of its constituentmicrolayers. Although only 6 ORUs (12 microlayers) are shown, the readerwill understand that a typical single packet reflective polarizer willcontain many more microlayers and ORUs in order to provide adequatereflectivity over the visible spectrum. For example, the total number ofmicrolayers in the single packet reflective polarizer may be less than500, or less than 400, or less than 350, or in a range from 200 to 500,or from 200 to 400, or from 200 to 350, or from 225 to 325, for example.The optical thickness of an ORU determines the wavelength at which theORU exhibits peak reflectivity. Careful control of the thicknesses ofthe ORUs in accordance with a desired layer thickness profile, whereinthe optical thicknesses of the ORUs gradually increase from one side ofthe packet (e.g. near the major surface 220 a) to the opposite side ofthe packet (e.g. near the thick layer 222), allows the packet ofmicrolayers to provide a broad reflectivity over the visible spectrumand over a desired range of observation angles, provided a sufficientnumber of ORUs are present in the packet.

An alternative approach, to more easily achieve the desired opticalperformance targets, is to design the multilayer optical film reflectivepolarizer to have more microlayers than can be practically incorporatedinto a single packet film. For this reason (or for other reasons),reflective polarizers are made in which the microlayers are divided orseparated into two or more distinguishable microlayer packets, with atleast one optically thick polymer material separating neighboringpackets. Such multiple packet reflective polarizers can be manufacturedin various ways. For example, the reflective polarizer can be made usingmultiple feedblocks (corresponding to the multiple packets) andcombining the packets from these feedblocks while the polymer materialsare still liquid, rather than using only one feedblock. See e.g. patentapplication publication US 2011/272849 (Neavin et al.). Alternatively,the reflective polarizer can be made using a layer multiplier device,e.g. as discussed in U.S. Pat. No. 5,882,774 (Jonza et al.) or U.S. Pat.No. 6,025,897 (Weber et al.). The layer multiplier device may forexample double or triple the number of microlayers and ORUs, producingtwo or three times (respectively) the number of packets in the finishedreflective polarizer. In still another approach, a multiple packetreflective polarizer can be made by laminating together two or moremultilayer optical film reflective polarizers that were each made with,for example, a single feedblock.

Drawbacks of multiple packet reflective polarizers tend to include (a)increased manufacturing cost because of the large number of layers andresulting high material cost, and (b) relatively large overall physicalthickness, which can be a significant disadvantage in some displayapplications. (The disclosed reflective polarizers desirably have athickness of less than 50 microns, or less than 40 microns, or in arange from 20 or 25 microns to 50 microns or from 20 or 25 microns to 40microns.) However, the larger number of microlayers allow the multiplepacket reflective polarizers to achieve display-quality opticalperformance targets even when such polarizers are oriented using astandard tenter, that is, even when the birefringent microlayers in suchreflective polarizers are biaxially birefringent. This is because themultiple packets can produce a spectral smoothing as described in patentapplication publication US 2013/0063818 (Weber et al.), resulting in areduced amount of off-axis color. Single packet reflective polarizerscannot take advantage of this spectral smoothing technique, and have asmaller margin of error with respect to layer thickness variability.

When discussing multilayer optical films made by coextruding numerouslayers of alternating polymer materials through a feedblock/die andorienting the film with a stretching operation, and the suitability ofsuch films in visual display applications, one aspect of the film ofpractical interest to the person of ordinary skill is the degree towhich the as-manufactured film is spatially uniform. This aspect of thefilm is of interest because it relates to how much of the manufacturedfilm can be used, versus how much must be disposed of, in the intendedapplication. This in turn affects the manufacturing yield and cost ofmanufacture, and it can also place size limitations on how large of apiece can be obtained or cut from a given web of film to fit a largedisplay system. In the case of optical films for use in LC displays, ahigh degree of spatial uniformity is desirable so that film-relatedartifacts are not noticeable in the displayed image.

A web of optical film 320 is shown schematically in FIG. 3. The film 320is manufactured on a film-making line and emerges from a tenter or otherstretching device, which is depicted schematically as element 309. Thefilm 320 has a longitudinal or down-web direction parallel to they-axis, as shown. The film 320 also has a transverse or cross-webdirection parallel to the x-axis, as shown. Two opposed longitudinaledges 320 a, 320 b define the longitudinal boundaries of the film 320.It is near these edges that the clip sets from the tenter or specializedstretching apparatus grasped the film during a previous orientationstep, after which the film 320 was trimmed to the edges 320 a, 320 b.Three film samples, intended for use as reflective polarizers in adisplay application or other desired application, are shown in thefigure: a film sample 321 a near the film edge 320 a, a film sample 321b near the film edge 320 b, and a film sample 321 c in a central portion(in relation to the transverse direction) of the film 320. These filmsamples or pieces are cut from the larger web or film 320 with a knife,slitter, or other suitable cutting implement. As a reflective polarizer,the optical film 320, and each of the film samples 321 a, 321 b, 321 c,has a block axis parallel to the x-axis, and a pass axis parallel to they-axis.

In an idealized situation, the film samples 321 a, 321 b, 321 c will allhave the same optical characteristics and properties. However, inpractice, the film 320 exhibits a certain amount of spatial variability.As a result, the layer thickness profile of the microlayer packet (andits corresponding spectral transmission and reflection characteristics)near an edge of the film 320 differs somewhat from the layer thicknessprofile (and corresponding spectral transmission and reflectioncharacteristics) in the central portion of the film. The amount ofchange in the spectral characteristics between the center and edge ofthe film is particularly significant for the type of multilayer opticalfilm of interest to the present application, namely, a reflectivepolarizer having only one packet of microlayers and oriented using astandard tenter such that birefringent microlayers in the film arebiaxially birefringent. This is because such films lack the spectralsmoothing that is provided by the multiple packets of other types ofreflective polarizers. See e.g. patent application publication US2013/0063818 (Weber et al.).

Elsewhere in this document we discuss optical properties such astransmission and reflection of certain polarizing films and laminates atspecific angles and polarization states. FIG. 4 is provided to assistthe reader in understanding some relevant directions, planes, andangles. In the figure, an optical body 412, which may for example be orcomprise a multilayer optical film configured as a reflective polarizer(such as a TOP reflective polarizer), or such a film laminated to anabsorbing polarizer and/or to another optical film or body, is shown inthe context of a Cartesian x-y-z coordinate system. As a polarizer, theoptical body 412 has a pass axis 413 and a block axis 414, whichcorrespond to the mutually perpendicular y- and x-axes, respectively.The z-axis corresponds to a thickness direction of the body 412, i.e.,an axis perpendicular to the plane of the body 412. Light that isnormally incident on the body 412 propagates parallel to the z-axis,characterized by a polar angle (θ) of zero. Such light is substantiallytransmitted by the body 412 if the light has a linear polarizationcomponent parallel to the pass axis 413, and is substantially blocked(reflected in the case of a reflective polarizer, absorbed in the caseof an absorbing polarizer) if the light has a linear polarizationcomponent parallel to the block axis 414.

For lack of an alternative term, “plane of incidence” is used herein torefer to the reference plane containing the surface normal direction andthe light propagation direction, both in cases where the light isincident on the film, and in cases where light is not incident on thefilm but instead is emerging from the film. Likewise, “incidence angle”is used to refer to the angle between the surface normal direction(z-axis) and the light propagation direction, both for light incident onthe film and for light emerging from the film, this angle alsocorresponding to the polar angle θ.

Two reference planes of incidence, 416 and 418, are included in thefigure: reference plane 416 contains the block axis 414 and the z-axis;and reference plane 418 contains the pass axis 413 and the z-axis. Twoobliquely incident light rays 415, 417 are shown in the figure. Ray 415lies in plane 416, and ray 417 lies in plane 418. The rays 415, 417 areobliquely incident because their directions of propagation formrespective non-zero polar angles θ with respect to the z-axis. For eachray 415, 417, the polarization state of the light ray can be resolvedinto two orthogonal components, represented in the figure as a pair oforthogonal double-headed arrows: a component whose polarization state isin the plane of incidence, referred to as “p-polarized”, and a componentwhose polarization state is perpendicular to the plane of incidence,referred to as “s-polarized”. Inspection of the figure reveals that thepolarization direction of p-polarized light for oblique ray 415 is notthe same as (and is not parallel to) the polarization direction ofp-polarized light for oblique ray 417. Similarly, the polarizationdirection of s-polarized light for oblique ray 415 is not the same as(and is not parallel to) the polarization direction of s-polarized lightfor oblique ray 417. Also apparent is that the p-polarized (“p-pol”)component of ray 415 is perpendicular to the pass axis 413 and partiallyaligned with the block axis 414, while the s-polarized (“s-pol”)component of ray 415 is parallel to the pass axis 413. The p-polcomponent of ray 417 is perpendicular to the block axis 414 andpartially aligned with the pass axis 413, while the s-pol component ofray 417 is parallel to the block axis 414. From this, one can see thatdepending on the direction of incidence, p-polarized light can beperpendicular to the pass axis in some cases and perpendicular to theblock axis in others, and s-polarized light can be parallel to the passaxis in some cases and parallel to the block axis in others.

The two oblique light rays 415, 417 are special cases of the moregeneral case of an arbitrary obliquely incident light ray, whicharbitrary oblique ray may have a plane of incidence that is parallel toneither plane 416 nor plane 418, i.e., parallel to neither the x-axisnor the y-axis. To fully characterize such an arbitrary oblique ray, weemploy an additional angle ϕ, referred to as an azimuthal angle. Theazimuthal angle ϕ is the angle, measured in the x-y plane, between thex-axis (i.e., the block axis) and the projection of such ray in the x-yplane, or between the x-axis (block axis) and the plane of incidence ofsuch ray. A value of ϕ=0 degrees corresponds to the plane 416, and avalue of ϕ=90 degrees corresponds to the plane 418.

Turning now to FIG. 5, we see there schematically illustrated selectedelements of an LC display system 500. The selected elements shown are aback absorbing polarizer 558 (which may be the same as or similar to theback absorbing polarizer 158 in FIG. 1), a multilayer optical filmreflective polarizer 520 (which may be the same as or similar to thereflective polarizer 142 of FIG. 1, or the reflective polarizer 220 ofFIG. 2), and a light diffusing layer 525 disposed on the front majorsurface of the reflective polarizer 520. Other components that would beincluded in the LC display system, such as an LC panel, a frontabsorbing polarizer, and a backlight, are omitted from the figure forsimplicity. The optical films lie generally in, or parallel to, the x-yplane. A first user or viewer 508 is located in front of the system 500and views the display at normal incidence, along a system optical axisparallel to the z-axis. A second user or viewer 509 is also located infront of the system 500, but views the display at an oblique angle.

The back absorbing polarizer 558 is assumed to be any of the absorbingpolarizers known in the art for their suitability in LC displays. Thepolarizer 558 has a pass axis and a block axis (not shown in FIG. 5),the polarizer being oriented such that the pass axis is parallel to they-axis, and the block axis is parallel to the x-axis. In contemporary LCdisplays, the back absorbing polarizer 558 is usually a high contrastpolarizer, greater than 1000 contrast ratio. In this regard, thecontrast of a polarizer for purposes of this document refers to a ratioof the polarizer's transmission for pass-state polarized light to thepolarizer's transmission for block-state polarized light, and, unlessotherwise specified, for light that is normally incident on thepolarizer, and whose wavelength is in the visible spectrum or within anyother useful wavelength range for the polarizer. An absorbing polarizeris said to have a high contrast if the contrast is at least 1000, or atleast 10,000 in some cases. Currently available absorbing polarizers mayhave a contrast in a range from 1000 to 100,000, or from 2,000 to10,000, for example.

The reflective polarizer 520 is assumed to be a TOP(tentered-one-packet) reflective polarizer as described above. FIG. 5depicts the polarizer 520 in a stand-alone configuration, in keepingwith the popular belief that TOP reflective polarizers are not suitablefor an on-glass configuration due to excessive off-axis color of thepolarizer. Thus, the reflective polarizer 520 is separated from theabsorbing polarizer 558 by an air gap 505. Furthermore, the reflectivepolarizer 520 is provided with a light diffusing layer 525 on one majorsurface thereof, the diffusing layer 525 being disposed between thereflective polarizer 520 and the absorbing polarizer 558. The lightdiffusing layer 525 scatters light into a cone or distribution of anglesas shown by incident light ray 506 and scattered light rays 507. Thescattering effectively mixes light rays that propagate through thereflective polarizer 520 in different directions to reduce or eliminatecolor associated with the reflective polarizer 520. The diffusing layer525 is assumed to have a relatively high haze, e.g. at least 40%, asmeasured using a Haze Guard Plus haze meter. The diffusing layer 525 maybe of any known type or design, for example, it may comprise glass orceramic beads or other particles immersed in a matrix of a differentrefractive index, or it may comprise a textured, faceted, or otherwisenon-smooth major surface at a polymer/air or polymer/polymer interface.

As already mentioned, we have found through investigation and testingthat, contrary to prevailing opinion, a properly designed and orientedTOP reflective polarizer can provide acceptable optical performance inan on-glass configuration, i.e., when laminated to a high contrast backabsorbing polarizer, and with no diffusing layer or structuretherebetween. (In some cases, however, a diffusing layer or structuremay be included that has a relatively low haze, e.g., less than 30%, orless than 20%, or less than 10% haze). Two examples of an on-glassconfiguration are shown in FIGS. 6 and 7.

In the schematic view of FIG. 6, a laminate 630 or optical body isshown, wherein a multilayer optical film reflective polarizer 620attaches to a back absorbing polarizer 658 by a transparent adhesivelayer 626. The reflective polarizer 620, the back absorbing polarizer658, and the adhesive layer 626 are all coextensive with each other, andthere is no air gap between the reflective polarizer 620 and theabsorbing polarizer 658. A viewer side of the laminate 630 is in thepositive z direction, thus, the back absorbing polarizer 658 may beconsidered to be in front of the reflective polarizer 620. Thereflective polarizer 620 may be the same as or similar to the reflectivepolarizer 520 described above. In fact, in the description that followswe assume the reflective polarizer 620 is a TOP reflective polarizer.The TOP reflective polarizer 620 may be a central portion of areflective polarizer web, see e.g. film sample 321 c in FIG. 3, or itmay be an edge portion, see e.g. film samples 321 a, 321 b.

The TOP reflective polarizer 620 has a pass axis 613 a, generallyparallel to the y-axis, and a block axis 614 a, generally parallel tothe x-axis. The number of ORUs in the single microlayer packet, and thethickness profile of those ORUs, provides the reflective polarizer 620with a high transmission for normally incident visible light polarizedparallel to the pass axis 613 a, and a low transmission (and highreflection, since transmission+reflection is about equal to 100% forthese low-absorption multilayer optical films) for normally incidentvisible light polarized parallel to the block axis 614 a. For example,the transmission of normally incident visible light polarized parallelto the pass axis 613 a may be at least 60%, or at least 70%, or at least80% when averaged over the visible wavelength range, and thetransmission of normally incident visible light polarized parallel tothe block axis 614 a may be less than 30%, or less than 20%, or lessthan 10%, when averaged over the visible wavelength range. Opticalperformance of the TOP reflective polarizer 620 for oblique p-polarizedlight, incident in a reference plane that contains the z-axis and thepass axis 613 a, is influenced by the unavoidable layer-to-layerrefractive index mismatches resulting from the biaxially birefringentnature of the birefringent microlayers in the film. For such obliquelight at a 60 degree polar angle of incidence, the transmission of thereflective polarizer 620 (by itself, in isolation from any absorbingpolarizer) has a value in a range from 70% to 90%, or from 70% to 85%,for at least some wavelengths from 450 to 700 nm; in some cases, thetransmission for such oblique light may be less than 90% throughout awavelength range from 400 to 500 nm.

The TOP reflective polarizer 620 may have an overall thickness of lessthan 50 microns, or less than 40 microns, or it may be in a range from20 to 50 microns, or in a range from 20 to 40 microns, or in a rangefrom 25 to 40 microns. The layer thickness profile of the ORUs in themicrolayer packet of the polarizer 620 may be tailored so that the colorshift of transmitted white light from normal incidence to any viewingangle up to and including a polar angle θ of 80 degrees is notobjectionable, as described further below. In particular, if we quantifythe color shift of such light by the change (A) from a first CIEchromaticity (a*, b*) coordinate at normal incidence to a second CIEchromaticity (a*, b*) coordinate at any angle up to θ=80 degrees, thensqrt((Δa*){circumflex over ( )}2+(Δb*){circumflex over ( )}2) isdesirably less than 3.5, more desirably less than 2.5, and mostdesirably less than 2.0.

The back absorbing polarizer 658, which has a pass axis 613 b and ablock axis 614 b, may be the same as or similar to the back absorbingpolarizer 558 described above. In fact, we assume the absorbingpolarizer 658 is a high contrast absorbing polarizer. The absorbingpolarizer 658 is oriented relative to the reflective polarizer 620 suchthat the pass axes 613 a, 613 b are substantially aligned, and the blockaxes 614 a, 614 b are also substantially aligned. For example, two suchsubstantially aligned axes may be characterized by an angular deviationof less than 1 degree, or less than 0.1 degrees.

The transparent adhesive layer 626 may be any suitable optical adhesive,for example, any of the Optically Clear Adhesive products available from3M Company, St. Paul, Minn. The refractive index of the adhesive layer626 is desirably reasonably close to the refractive index of theexterior surface of the absorbing polarizer 658 and the refractive indexof the exterior surface of the reflective polarizer 620, to avoidFresnel reflection at the polymer/adhesive interfaces of those films.The adhesive layer 626 preferably provides a permanent bond between theabsorbing polarizer 658 and the reflective polarizer 620.

The laminate 630 may consist (only) of, or it may consist essentiallyof, the reflective polarizer 620, the absorbing polarizer 658, and theadhesive layer 626. In some embodiments, the laminate 630, and each ofthese three components, does not incorporate any significantidentifiable light diffusing layer or structure, such as beads or otherparticles of different refractive index, or a textured or othernon-smooth major surface. The laminate 630 may thus be devoid of anysuch light diffusing layer or structure. In cases where the laminate 630does include such a diffusing layer or structure, that layer may bebetween the reflective polarizer 620 and the absorbing polarizer 658, oron the side of the reflective polarizer 620 opposite the absorbingpolarizer 658, or within the reflective polarizer 620, or within theabsorbing polarizer 658. The foregoing statements are made with therecognition that even ideal, flat optical films and layers withexceptional optical clarity may exhibit a minute but measureable amountof optical scattering or diffusion. Thus, for clarity, we may establisha minimal threshold below which the layer or structure at issue may beconsidered, from a practical standpoint and for the purposes of thepresent document, to have no light diffusion. We set this minimal lightdiffusion threshold at a haze value of 5%, or 4%, or 3%, or 2%, or 1%,as measured using a Haze Guard Plus haze meter from BYK-Gardiner, SilverSprings, Md., according to a suitable procedure such as that describedin ASTM D1003.

Optical films are often sold and/or shipped with a temporary polymericrelease liner on both sides to protect the major surfaces of the filmfrom scratches or other damage. Such release liners can be easilyremoved from the product by peeling. The release liners can incorporatedyes, pigments, or other agents, including light diffusing agents, sothey can be easily seen or detected by the user. Such temporary releaseliners may be applied to the outer surfaces of the laminate 630 as well.However, such release liners are distinguishable from, and need not beconsidered part of, the laminate 630. Thus, to the extent such releaseliners are present on the laminate 630 (or on other laminates disclosedherein, including laminate 730 below) and have a substantial lightdiffusion property, it can still be correct to state that the laminatedoes not incorporate any significant light diffusing layer or structure.

The reader should be cautioned, however, that in some cases it can bedesirable to include one or more moderate diffusing layers or structuresbetween the reflective polarizer 620 and the absorbing polarizer 658,such moderate diffusing layers or structures having an amount of hazethat is significant, i.e., greater than the above-mentioned minimallight diffusion threshold, yet smaller than high haze diffuserstypically found in stand-alone configurations such as that of FIG. 5. Adiffusing layer or structure may for example be included between the TOPreflective polarizer 620 and the high contrast absorbing polarizer 658that has a relatively low haze, e.g., less than 30%, or less than 20%,or less than 10% haze.

The microlayers and ORUs in the microlayer packet of the disclosed TOPreflective polarizers have physical thicknesses, optical thicknesses, orboth that are carefully tailored, and properly oriented, to provide athickness profile that gives the reflective polarizer not only lowtransmission (high reflectivity) for the block state polarization andhigh transmission (low reflectivity) for the pass state polarization,over the visible wavelength range of interest, and for both normalincidence and highly oblique incidence, but also low transmitted colorfor the highly oblique light, particularly at intermediate azimuthalangles ϕ, e.g. where ϕ is in a range from 15 to 45 degrees. Undesiredcolor at high oblique angles in the pass state of the polarizer (whichis closely associated with the white state of the display in which theTOP reflective polarizer is located) is reduced by positioning thickerORUs in the microlayer packet closer than thinner ORUs to the highcontrast absorbing polarizer, and by satisfying a condition describedfurther below that involves an intrinsic-bandwidth based boxcar averageof the physical thickness profile (IB-smoothed thickness profile), andthe presence of an ORU(450), an ORU(600), and an ORU(645), and ensuringthat the IB-smoothed thickness profile has a first average slope over arange from ORU(450) to ORU(600), and a second average slope over a rangefrom ORU(600) to ORU(645), and the ratio of the second average slope tothe first average slope is no more than 1.8. By satisfying thiscondition, the TOP reflective polarizer, and the laminate of which it isa part, can impart an amount of color—to white light passing through itat highly oblique angles—that is so small that, for a displayincorporating such a polarizer or laminate, the perceived color of thewhite state of such display at such highly oblique angles is acceptablyclose to a neutral white or target white color.

Another laminate 730 or optical body is shown in FIG. 7. The laminate730 may be the same as or similar to the laminate 630 as describedabove, except that two additional layers have been added. Thus, thelaminate 730 includes a high contrast back absorbing polarizer 758, aTOP reflective polarizer 720, and an adhesive layer 726 that bonds theabsorbing polarizer 758 to the reflective polarizer 720. These elementsmay be the same as or similar to corresponding elements of the laminate630, and they form an optical body or structure 730′ which may thus bethe same as or similar to the laminate 630, except that the front ofstructure 730′ is attached to additional layers. In particular, thefront major surface of the high contrast back absorbing polarizer 758 isbonded to a glass layer 754 through an adhesive layer 728. The adhesivelayer 728 may be the same as or similar to the adhesive layer 726. Theglass layer may be the back or rear panel plate of a liquid crystalpanel, such as the panel plate 154 b of the LC panel 150, describedabove.

The laminate 730 may consist (only) of, or it may consist essentiallyof, the elements 720, 726, 758, 728, and 754 as described above. Similarto the laminate 630, the laminate 730 and each of its componentspreferably does not incorporate any significant identifiable lightdiffusing layer or structure, such as beads or other particles ofdifferent refractive index, or a textured or other non-smooth majorsurface. The laminate 730 may thus be devoid of any such light diffusinglayer or structure. In cases where the laminate 730 does include such adiffusing layer or structure, that layer may be between the reflectivepolarizer 720 and the absorbing polarizer 758, or on the side of thereflective polarizer 720 opposite the absorbing polarizer 758, or withinthe reflective polarizer 720, or within the absorbing polarizer 758. Asdiscussed above, even ideal, flat optical films and layers withexceptional optical clarity may exhibit measureable optical scattering,and we may establish a minimal threshold below which the layer orstructure at issue may be considered to have no light diffusion forpurposes of the present document. Suitable threshold values are givenabove. Furthermore, in some cases it can be desirable to include one ormore diffusing layers or structures between the reflective polarizer 720and the absorbing polarizer 758 that have a small but significant amountof haze, e.g., less than 30%, or less than 20%, or less than 10% haze.

The layer thickness profile used in the disclosed TOP reflectivepolarizers, such as those of the FIGS. 6 and 7 laminates, warrants someadditional discussion. As already mentioned, the microlayers in themicrolayer packet are organized into optical repeat units (ORUs), andthe optical thicknesses of the ORUs (and microlayers) are tailored toprovide, for light throughout the visible spectrum, a high broadbandreflectivity for light of the block polarization, and a high broadbandtransmission (low reflectivity) for light of the pass polarization, bothat normal incidence and over a desired range of oblique incidence anglesand directions. This is accomplished by tailoring the thickness profileof the ORUs along the thickness direction (z-axis) of the film to be amonotonic, or near-monotonic, function, with thinner ORUs locatedgenerally at one side of the packet (referred to here as the thin side),and thicker ORUs located generally at the opposite side of the packet(referred to here as the thick side).

In order to reduce the undesirable perceived off-axis transmitted colorof the disclosed films, it is helpful to (a) orient the reflectivepolarizer such that the thick side of the microlayer packet faces theabsorbing polarizer (and thus also the observer and the LC panel), andthe thin side of the microlayer packet faces away from the absorbingpolarizer (and thus towards the backlight of a display system), and (b)tailor the ORU thickness profile to be smoothly varying in such a waythat an IB-smoothed thickness profile of the microlayer packet has afirst average slope over a range from an ORU(450) to an ORU(600), and asecond average slope over a range from the ORU(600) to an ORU(645), andensuring that the ratio of the second average slope to the first averageslope is no more than 1.8. By satisfying this condition, the TOPreflective polarizer, and the laminate of which it is a part, can impartan amount of color—to white light passing through it at highly obliqueangles—that is so small that, for a display incorporating such apolarizer or laminate, the perceived color of the white state of suchdisplay at such highly oblique angles is acceptably close to a neutralwhite or target white color.

The discussion above relating to the ORU thickness profile and itscharacteristics that we have found can be tailored to keep the coloreffects of the TOP reflective polarizer, and of a laminate incorporatingsame, to acceptably low levels will now be explained in further detailand with the aid of numerous embodiments as well as some examples. Webegin the more detailed discussion with reference to FIGS. 8A and 8B,which relate to a simplified ORU physical thickness profile. Thesimplified thickness profile allows us to more easily describe conceptssuch as smoothing of the thickness profile using boxcar averaging, andresonant wavelengths for the boxcar-averaged (smoothed) thicknessprofile at a given optical geometry, for each ORU. After this, numerousTOP reflective polarizer embodiments, and their modeled color-relatedperformance at highly oblique angles, are discussed and compared inconnection with FIGS. 9 through 30C. Finally, an example TOP reflectivepolarizer is discussed in connection with FIGS. 31 through 32C, and twocomparative example TOP reflective polarizers are discussed inconnection with FIGS. 33 through 36C. Unless otherwise indicated, all ofthe embodiments include an ORU physical thickness profile having agradient that provides a wide band reflectivity for normally incidentlight polarized along the block axis, and the ORUs have resonantwavelengths as a function of the physical thickness profile and opticalgeometry.

FIGS. 8A and 8B illustrate in simplified fashion the concepts of an ORUphysical thickness profile, a smoothed physical thickness profile byboxcar averaging (e.g. an intrinsic-bandwidth (IB) based boxcaraverage), and oblique angle resonant wavelengths. In FIG. 8A, thephysical thickness profile of a hypothetical TOP reflective polarizer ispresented in a graph that plots ORU thickness against ORU number. Forgenerality, the vertical axis of the graph is not provided withnumerical markings, but the reader will understand that thicknessincreases linearly in the direction of the axis arrow. The horizontalaxis is simply a count of the number of ORUs starting at a first end ofthe microlayer packet. (As such, the horizontal axis is also closelyrelated to a physical position or depth within the reflective polarizingfilm relative to the first end of the packet.) Inspection of that axisreveals that the hypothetical polarizer has exactly 15 ORUs. Each ORUmay consist of two adjacent microlayers, e.g. as shown above in FIG. 2.A point P1 represents the physical thickness of the first ORU, a pointP2 represents the physical thickness of the second ORU, and so forth,and a point P15 represents the physical thickness of the fifteenth (andlast) ORU in the packet of microlayers forming the illustrated TOPreflective polarizer. The collection of points P1 through P15 is the ORUphysical thickness profile of the packet of microlayers of thereflective polarizer. As drawn, this thickness profile is monotonicallyincreasing, and substantially linear as a function of ORU number. ORUsproximate the fifteenth (last) ORU have an average physical thicknessgreater than that of ORUs proximate the first ORU.

For any given optical geometry, each ORU produces a reflectivityspectrum characterized by (1) a peak or maximum reflectivity, and (2) aspectral breadth or width (e.g. as measured by thefull-width-at-half-maximum of the reflectivity spectrum), which we referto as the intrinsic bandwidth. The aggregate of the spectralreflectivities of all the ORUs in the microlayer packet thensubstantially yield the reflectivity of the TOP reflective polarizer asa whole. The peak reflectivity of a given ORU occurs at a wavelengththat is referred to as a resonant wavelength (for that ORU); however,the peak reflectivity, and thus the resonant wavelength, changes as afunction of optical geometry. For normally incident light, the resonantwavelength equals one-half of the optical thickness of the ORU, whereoptical thickness differs from physical thickness as explained in detailabove. At oblique angles, the resonant wavelength is less than theresonant wavelength at normal incidence, and furthermore it is ingeneral different for s-polarized light and p-polarized light. Theintrinsic bandwidth of an ORU reflectance spectrum is also influenced tosome extent by optical geometry, but also by other factors such as therefractive indices of the microlayers in the packet, and the refractiveindex differences between such microlayers.

The fact that each ORU in the packet has a reflectivity with a non-zerointrinsic bandwidth means that the reflectivity of the reflectivepolarizer as a whole at a given specific wavelength (and for a specificoptical geometry) is attributable to not only the ORU whose resonantwavelength equals the specific wavelength, but also to neighboring ORUswhose resonant wavelengths are close to (in terms of the intrinsicbandwidths of the ORU reflection spectra) the specific wavelength. Forexample, if we assume the ORU represented by point P10 has a resonantwavelength of precisely λ10 at a given optical geometry, and then weconsider the reflectivity of the reflective polarizer (and packet) as awhole at that wavelength λ10 (and at the given optical geometry), suchlatter reflectivity may be attributable not only to the ORU of pointP10, but also to several of its nearest neighboring ORUs on both sides,as indicated by the group G10 in FIG. 8A. In the case of thereflectivity at a wavelength corresponding to a resonant wavelength ofan ORU at or near one of the two ends of the microlayer packet, such asthe ORU for point P1 or for point P15 in FIG. 8A, nearest neighbor ORUsmay exist only on one side of the subject ORU, which results inone-sided or unbalanced ORU groupings, such as the group G1 and thegroup G15 in FIG. 8A.

The foregoing phenomenon of overlapping reflectivity bands forneighboring ORUs leads us to define a smoothed ORU thickness profile,obtained by averaging, at each of the 15 ORUs, the thickness of thesubject ORU and of its neighboring ORUs, if any, on each side of thesubject ORU. We refer to this technique as boxcar averaging. The actualnumber of neighboring ORUs to include on each side of the subject ORUwill depend on the intrinsic bandwidths of the ORUs in the microlayerpacket and on other factors, but for purposes of this simplifiedexample, we include 2 ORUs (to the extent they exist) on each side ofthe subject ORU. The result of this intrinsic-bandwidth (IB) basedboxcar averaging is illustrated in FIG. 8A by the small open-circlemarks provided at each of the 15 ORUs, namely, A1, A2, A3, and so forth,through A15. To the extent the original ORU thickness profile asrepresented by points P1 through P15 is strictly linear, the marks A3through A13 will also be linear, and will coincide with their respectivepoints P3 through P13, due to the symmetrical nature of the boxcaraveraging for those ORUs. However, for the ORUs at and near the ends ofthe packet, the IB-smoothed thickness profile deviates from the originalthickness profile, as shown by the non-registration of the open circlesA1, A2, A14, A15 with their respective points P1, P2, P14, P15, due tothe asymmetrical nature of the boxcar averaging for those ORUs.

Following these computations, we next compute the resonant wavelengthfor each of the 15 ORUs based on the IB-smoothed thickness profile andon a sufficiently oblique optical geometry, which we select as beingp-polarized light incident in the x-z plane (see e.g. FIG. 4 above) at apolar angle (θ) of 80 degrees. This computation may employ a Berriman4×4 matrix multilayer optical response calculation engine that utilizesas inputs the IB-smoothed thickness profile and other parameters, suchas the (wavelength-dependent) refractive index values nx, ny, nz foreach of the microlayers in the ORUs (see the discussion of FIG. 2above), as well as the specified oblique optical geometry. With thisinformation, the resonant wavelength at the specified oblique anglegeometry can be calculated for each of the 15 ORUs. The result isplotted in a graph of resonant wavelength versus ORU number in FIG. 8B.The Berriman methodology can also be used to calculate the spectralreflectivity and spectral transmission of the TOP reflective polarizerand of such polarizer laminated to a high contrast absorbing polarizer.

For generality, the vertical axis of the graph is not provided withnumerical markings, but the reader will understand that resonantwavelength increases linearly in the direction of the axis arrow. Thehorizontal axis is simply a count of the number of ORUs starting at thefirst end of the microlayer packet, just as in the graph of FIG. 8A. Thex-shaped points, labeled W1, W2, W3, and so forth through W15, representthe resonant wavelengths calculated at each of the ORUs.

Having now performed the foregoing calculations, we are prepared toengage in an analysis to determine if the (hypothetical simplified) TOPreflective polarizer satisfies conditions that will promote lowperceived color in the transmission of the reflective polarizer and of alaminate of the reflective polarizer with a high contrast absorbingpolarizer. As part of this analysis, we determine if any of the ORUs inthe microlayer packet satisfy both of the following conditions: theresonant wavelength at the specified oblique optical geometry(p-polarized light incident in the x-z plane θ=80 degrees), and for theIB-smoothed thickness profile, is at least 450 nm; and all ORUs disposedon a side of the ORU that includes the first ORU have resonantwavelengths (under the same conditions) less than 450 nm. If so, werefer to this ORU as ORU(450). (Preferably, the resonant wavelength ofthe ORU(450) is less than 455 nm, for the IB-smoothed thickness profileat the specified oblique optical geometry.) Similarly, we determine ifany of the ORUs in the microlayer packet satisfy both of the followingconditions: the resonant wavelength at the specified oblique opticalgeometry (p-polarized light incident in the x-z plane θ=80 degrees), andfor the IB-smoothed thickness profile, is at least 600 nm; and all ORUsdisposed on a side of the ORU that includes the first ORU have resonantwavelengths (under the same conditions) less than 600 nm. If so, werefer to this ORU as ORU(600). (Preferably, the resonant wavelength ofthe ORU(600) is less than 605 nm, for the IB-smoothed thickness profileat the specified oblique optical geometry.) In like fashion, wedetermine if any of the ORUs in the microlayer packet satisfy both ofthe following conditions: the resonant wavelength at the specifiedoblique optical geometry (p-polarized light incident in the x-z planeθ=80 degrees), and for the IB-smoothed thickness profile, is at least645 nm; and all ORUs disposed on a side of the ORU that includes thefirst ORU have resonant wavelengths (under the same conditions) lessthan 645 nm. If so, we refer to this ORU as ORU(645). (Preferably, theresonant wavelength of the ORU(645) is less than 650 nm, for theIB-smoothed thickness profile at the specified oblique opticalgeometry.) Note that the ORU(400), ORU(600), ORU(645) are defined at ahighly oblique optical geometry, which means that their characteristicsfor normally incident light will be substantially different than theircharacteristics at the oblique geometry. For example, the ORU(645) willlikely have a resonant wavelength for normally incident light that iswell into the near-infrared portion of the electromagnetic spectrum.

If the TOP reflective polarizer contains all three of the ORU(400),ORU(600), and ORU(645), then we perform further analysis on theIB-smoothed thickness profile (see e.g. points A1 through A15 in FIG.8A), and in particular, on an average slope of that profile. We comparea first average slope of the IB-smoothed thickness profile over a shortwavelength range, namely, from ORU(400) to ORU(600), to a second averageslope of the same profile over a longer wavelength range, namely, fromORU(600) to ORU(645). Low transmitted color for the reflectivepolarizer, for a laminate of the polarizer and a high contrast absorbingpolarizer (provided the reflective polarizer is oriented such that anend of the microlayer packet having thicker ORUs is adjacent to or facesthe absorbing polarizer), and for the white state of a displayincorporating the laminate, are promoted when a ratio of the secondaverage slope to the first average slope is no more than 1.8.

The reader is cautioned that although this analysis makes use of aresonant wavelength as calculated at a particular highly oblique opticalgeometry, namely, for p-polarized light incident in the x-z plane at apolar angle θ=80 degrees, the low color output is by no means limited tothat geometry. Stated differently, if the reflective polarizer, and thelaminate of which it is a part, is tailored as set forth in the aboveanalysis, low color transmission is achieved not only at the particularhighly oblique optical geometry used in the analysis, but also at otherhighly oblique geometries, including at intermediate azimuthal angles ϕbetween 0 and 90 degrees (for polar angles θ of at least 80 degrees),and for other polarization states.

FIG. 9 and its related FIGS. 10A through 17C demonstrate the applicationof these principles to a number of related (modeled) embodiments of aTOP reflective polarizer and laminates thereof with a high contrastabsorbing polarizer. The transmission and reflection spectra of thereflective polarizer were computed using a Berriman 4×4 matrixmultilayer optical response calculation engine. The input parameters forthe calculations included a layer thickness profile of ORUs, andwavelength-dependent refractive index values for the birefringentmicrolayers and the isotropic microlayers that make up the microlayerpacket and the ORUs.

In these embodiments, the TOP reflective polarizer has exactly 152 ORUs.Each ORU includes only two microlayers, one of which is biaxiallybirefringent, and the other of which is isotropic, with an f-ratio of0.5. The birefringent microlayer is assigned a refractive index set (nx,ny, nz) that is based on measured data for a uniaxially stretched LowMelt Point PEN (LmPEN). With regard to the composition of LmPEN, thediol is 100% ethylene glycol, while the diacid is 10 mol % teraphthalicacid and 90 mol % naphthalene dicarboxylic acid. The isotropicmicrolayer is assigned an isotropic refractive index (Niso) based onmeasured data for an amorphous blend of PETg GN071 (Eastman Chemicals,Knoxville, Tenn.) and LmPEN at the weight fraction of 58% and 42%,respectively. The refractive indices of these materials as used in ourcomputational modeling are shown in Table 1:

TABLE 1 LmPEN 58% PETg/42% LmPEN wavelength nx ny nz Niso 450 nm 1.9061.665 1.575 1.653 550 nm 1.836 1.622 1.547 1.611 633 nm 1.810 1.6071.538 1.595

Inspection of the table reveals that nx is greater than Niso, providinga large refractive index difference for electric fields along thex-axis. The value of ny is approximately equal to Niso. The value of nzis less than Niso, providing a refractive index difference forp-polarized light for non-normal incidence angles. Based on theteachings of the SCIENCE paper referenced below, the combination ofbirefringent and isotropic refractive indices shown in Table 1 willresult in increasing interfacial reflectivity, and increasing reflectionband power, for increasing incidence angle θ and for both s- andp-polarized light.

In FIG. 9, we have a graph of eight different but related ORU physicalthickness profiles 961, 962, 963, 964, 965, 966, 967, and 968, any ofwhich may be readily employed in a TOP reflective polarizer. Thethickness profile 961 is linear in form, i.e. of constant slope, fromthe first ORU (#1), whose physical thickness is about 120 nm, to thelast ORU (#152), whose physical thickness is about 265 nm. The otherthickness profiles 962-968 are identical to the thickness profile 961from ORU #1 through ORU #105, but then at ORU #105 they undergo astep-change in the slope of thickness profile from ORU #105 to ORU #152.The step change in slope is smallest for profile 962 and largest forprofile 968, as shown in the figure.

Each of these ORU thickness profiles is then analyzed as describedabove, by calculating from the ORU thickness profile:

-   -   an IB-smoothed thickness profile, where, for purposes of the        boxcar average, the number of ORUs that we include on each side        of the subject ORU is 10 (if present), to take into account the        intrinsic bandwidth of the ORU reflection bands for the modeled        refractive indices; and    -   the resonant wavelength of the IB-smoothed thickness profile at        each ORU, for the highly oblique optical geometry of p-polarized        light incident in the x-z plane at a polar angle θ of 80        degrees; and then    -   determining if the microlayer packet includes an ORU(450), an        ORU(600), and an ORU(645) as described above; and if so, then    -   calculating a first average slope of the IB-smoothed thickness        profile from the ORU(450) to the ORU(600), and calculating a        second average slope of the IB-smoothed thickness profile from        the ORU(600) to the ORU(645); and    -   calculating a ratio (“slope ratio”) of the second average slope        to the first average slope.    -   Note, if the microlayer packet does not include any one of the        ORU(450), the ORU(600), or the ORU (645), the aforementioned        slope ratio is undefined.

Besides this, we also use the Berriman methodology to calculate thecolor response of a laminate of the modeled TOP reflective polarizerwith an aligned absorbing polarizer having a contrast ratio of 10,000,with no air gap or diffusing material therebetween, and with thereflective polarizer oriented with the thicker microlayers (and ORUs)closer than the thinner microlayers (and ORUs) to the absorbingpolarizer, for a range of highly oblique optical geometries as definedby a range of polar angles θ from 45 to 85 degrees in 5 degreeincrements, and for azimuthal angles ϕ of 15, 25, 35, and 45 degrees.(Angles such at these are often susceptible to undesired color or colorvariation in the white state of standard display panels.) For each suchgeometry, for example, (θ=50 deg, ϕ=35 deg), the computational modellaunches an unpolarized input light beam towards the laminate at thespecified direction, such input beam being incident from air upon thethinnest-layer-end of the microlayer packet, i.e., on the ORU #1. Thespectral content of the input beam is modeled as a standard displaywhite LED, filtered through red, green, and blue color filters, typicalof an LC display. The Berriman methodology then calculates the outputbeam as transmitted through the laminate (both the TOP reflectivepolarizer and the aligned high contrast absorbing polarizer), andcalculates the spectral content of the output beam. Comparing thecomputed output beam to the input beam yields a color response for thelaminate at the particular geometry. We quantify the color response interms of the well-known CIE (L*, a*, b*) color coordinates. The generalcolor response performance of the laminate (and reflective polarizer)can be assessed by evaluating the computed (a*, b*) values across themodeled range of azimuth and polar angles.

Thus, in FIG. 10A, a compound graph has a single horizontal axis thatrepresents ORU number, a left-hand-side (LHS) vertical axis thatrepresents ORU (physical) thickness in nanometers (nm), and aright-hand-side (RHS) vertical axis that represents resonant wavelengthin nanometers (nm). The use of dual vertical axes allows us to plot bothORU thickness (using the LHS axis) and resonant wavelength (using theRHS axis) against ORU number on the same graph. The curves 961 and 961Aare plotted against the LHS vertical axis, and the curve 961W is plottedagainst the RHS vertical axis.

The curve 961 is identical to the ORU physical thickness profile 961 ofFIG. 9, having a constant slope from ORU #1 to ORU #152. (In practice,if an actual multilayer film sample is provided, the ORU physicalthickness profile is typically measured using an Atomic Force Microscopy(AFM) system, designed for multilayer polymer characterizations, andsome averaging or smoothing of the raw data from the AFM may be neededto obtain an accurate result.)

The curve 961A is the intrinsic-bandwidth (IB) based boxcar average ofthe curve 961. For any of the ORUs #11 through 142, the correspondingpoint on the averaged or smoothed curve 961A is derived by calculatingthe average thickness (from curve 961) for the group of ORUs consistingof the given ORU, and the 10 ORUs immediately adjacent to the given ORUon the left, and the 10 ORUs immediately adjacent to the given ORU onthe right. The averages for these ORUs #11 through 142 are thus derivedfrom groups of 21 ORUs. For ORUs at or near the ends of the microlayerpacket, i.e., for ORUs #1 through 10 and 143 through 152, fewer than 21ORUs are used in the group average, because, for these ORUs, fewer than10 ORUs are available on the left or on the right of the given ORU. A10-ORU-per-side (21-ORU group except at or near the ends) boxcar averageis appropriate and representative of the reflection/transmissionbehavior of the microlayer thickness profile for the disclosedembodiments, based on the intrinsic bandwidth of resonant ¼ wavelengthORUs, of the variety that dominate the transmission properties of amicrolayer thickness profile packet, in the critical viewer angle-rangeof interest. The intrinsic bandwidth of an ORU having ¼ wavelengthmicrolayers, with refractive indices as shown above in Table 1, isapproximately 10%. This implies that for any given wavelength,approximately 10 ORUs that are thicker, near-neighbors, and 10 ORUs thatare thinner, near-neighbors of the central ORU, will participate increating a reflection response at that given wavelength; hence our useof terms such as 21-ORU boxcar average reflector group.

The IB-smoothed thickness profile, evaluated at any given ORU,preferably encompasses substantially only those ORUs that coherentlycontribute to a reflectivity of the packet at the resonant wavelength ofthe given ORU. In the embodiments of primary interest in the discussionat hand, this means the IB-smoothed thickness profile encompasses 10nearest-neighbor ORUs on each side of a given ORU. In other embodiments,however, the IB-smoothed thickness profile may encompass a differentpredetermined number of nearest neighbor ORUs on each side the given ORUas a result of a different intrinsic bandwidth due to, for example,substantially different refractive indices, and refractive indexdifferences, compared to the embodiments of primary interest. In suchcases, the predetermined number of ORUs (on each side of the given ORU)may be no more than 20, but at least 5.

The plotted values of the 21-ORU boxcar average are calculated beginningat ORU #11, near the thin end of the microlayer packet, and continuingto the ORU #142. Then, from ORU #143 to the last ORU #152 on the thickend of the packet, the group of ORUs in the boxcar average diminishesfrom 20 down to 11 as the number of ORUs available on the rightdiminishes. Each of these boxcar-averaged ORU values represent a groupof microlayers that coherently act to create a reflection band at thewavelength that is appropriate to its optical phase thickness, which inturn, depends on the polarization state of the incident light, therefractive index of the external medium from which the incident lightemanates, and the and the azimuth and polar angle of the incident light.A detailed discussion of these properties for multilayered stacks ofbiaxial materials, is found in the paper “Giant Birefringent Optics inMultilayer Polymer Mirrors”, SCIENCE vol. 287, pp. 2451-2456 (Mar. 31,2000).

The curve 961W in FIG. 10A plots the resonant wavelength (in nanometers)of each boxcar-averaged reflector group, at a highly oblique opticalgeometry characterized by θ=80 degrees, ϕ=0 degrees, and for p-polarizedlight with the x-z plane as its plane of incidence. That is, any pointon the curve 961W at a given ORU is the resonant wavelength for theIB-smoothed thickness profile 961A at the given ORU, and at thespecified oblique optical geometry. Also plotted is a reference line λcat a critical wavelength, the critical wavelength selected to be 645 nmfor the disclosed embodiments. Inspection of the graph reveals that thecurve 961W encompasses the resonant wavelengths 400 nm, 600 nm, and 645nm. Consequently, the IB-smoothed thickness profile 961A includes anORU(400), an ORU(600), and an ORU(645).

FIG. 10B graphs the slope of the IB-smoothed thickness profile 961A fromFIG. 10A as a function of resonant wavelength as defined by the curve961W. That is, instead of scaling the horizontal axis of FIG. 10Baccording to the ORU number, it is scaled according to the resonantwavelength (at the specified oblique optical geometry) of each of theORUs. The calculated slope simply equals the rise over the run of thecurve 961A in FIG. 10A. This calculated slope is shown in FIG. 10B ascurve 961S, which has a discontinuity at about 665 nm due to the abruptchange in slope of the curve 961A near ORU #141. In FIG. 10B, thereference line λc appears vertically at the critical angle of 645 nm.Also included in FIG. 10B is a first region 1001, which extends from 450nm (corresponding to ORU(450)) to 600 nm (corresponding to ORU(600)),and a second region 1002, which extends from 600 nm to 645 nm(corresponding to ORU(645)).

The slope ratio discussed above involves computing a first average ofthe slope, i.e. of the curve 961S, over the range of the first region1001, and computing a second average of the slope (curve 961S) over therange of the second region 1002. In this embodiment, inspection of FIG.10B reveals that curve 961S is substantially flat over these tworegions, thus, the first and second averages are substantiallyidentical. For this embodiment, therefore, a ratio of these averages(the slope ratio) yields a value of 1.0.

FIG. 10C graphs the color response for the embodiment of FIGS. 10A and10B, as explained in detail above, using the (dimensionless) CIE a* andb* color coordinates. In brief, Berriman methodology was used tocalculate the color response of a laminate of the modeled TOP reflectivepolarizer with an aligned absorbing polarizer having a contrast ratio of10,000, with no air gap or diffusing material therebetween, and with thereflective polarizer oriented with the thicker microlayers (and ORUs)closer than the thinner microlayers (and ORUs) to the absorbingpolarizer. The color response was calculated for highly oblique opticalgeometries for polar angles θ ranging from 45 to 85 degrees in 5 degreeincrements, and for azimuthal angles ϕ of 15, 25, 35, and 45 degrees.The results are grouped in terms of azimuthal angle, with the curve ϕ15showing the color response for ϕ=15 deg over the range of polar angles,and the curve ϕ25 showing the color response for ϕ=25 deg over the rangeof polar angles, and the curve ϕ35 showing the color response for ϕ=35deg over the range of polar angles, and the curve ϕ45 showing the colorresponse for ϕ=45 over the range of polar angles. A reference circlelabeled C is provided to indicate the same scale when comparing (a*,b*)graphs presented herein. Unless otherwise noted, the circle C has adiameter of 3.0 (unit-less dimension) in all such graphs.

The same methodology as described above for the ORU physical thicknessprofile 961, and as shown in FIGS. 10A, 10B, and 10C, was then repeatedfor each of the other related ORU physical thickness profiles shown inFIG. 9.

Thus, for the profile 962: FIG. 11A is a compound graph in which curve962 is identical to the profile 962, curve 962A is the IB-smoothedthickness profile of curve 962, the curve 962W is the resonantwavelength for the IB-smoothed thickness profile at the specifiedoblique optical geometry, and reference line λc marks the criticalwavelength at 645 nm; FIG. 11B graphs as curve 962S the slope of theIB-smoothed thickness profile 962A as a function of resonant wavelengthas defined by curve 962W, and provides a first region 1101 from 450 to600 nm and a second region 1102 from 600 to 645 nm, and the referenceline λc at 645 nm; and FIG. 11C graphs the color response in a*, b*color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogousmeanings to those of FIG. 10C, and with a reference circle C.

For the profile 963: FIG. 12A is a compound graph in which curve 963 isidentical to the profile 963, curve 963A is the IB-smoothed thicknessprofile of curve 963, the curve 963W is the resonant wavelength for theIB-smoothed thickness profile at the specified oblique optical geometry,and reference line λc marks the critical wavelength at 645 nm; FIG. 12Bgraphs as curve 963S the slope of the IB-smoothed thickness profile 963Aas a function of resonant wavelength as defined by curve 963W, andprovides a first region 1201 from 450 to 600 nm and a second region 1202from 600 to 645 nm, and the reference line λc at 645 nm; and FIG. 12Cgraphs the color response in a*, b* color coordinates of the reflectivepolarizer-absorbing polarizer laminate, with the curves ϕ15, ϕ25, ϕ35,and ϕ45 having analogous meanings to those of FIG. 10C, and with areference circle C.

For the profile 964: FIG. 13A is a compound graph in which curve 964 isidentical to the profile 964, curve 964A is the IB-smoothed thicknessprofile of curve 964, the curve 964W is the resonant wavelength for theIB-smoothed thickness profile at the specified oblique optical geometry,and reference line λc marks the critical wavelength at 645 nm; FIG. 13Bgraphs as curve 964S the slope of the IB-smoothed thickness profile 964Aas a function of resonant wavelength as defined by curve 964W, andprovides a first region 1301 from 450 to 600 nm and a second region 1302from 600 to 645 nm, and the reference line λc at 645 nm; and FIG. 13Cgraphs the color response in a*, b* color coordinates of the reflectivepolarizer-absorbing polarizer laminate, with the curves ϕ15, ϕ25, ϕ35,and ϕ45 having analogous meanings to those of FIG. 10C, and with areference circle C.

For the profile 965: FIG. 14A is a compound graph in which curve 965 isidentical to the profile 965, curve 965A is the IB-smoothed thicknessprofile of curve 965, the curve 965W is the resonant wavelength for theIB-smoothed thickness profile at the specified oblique optical geometry,and reference line λc marks the critical wavelength at 645 nm; FIG. 14Bgraphs as curve 965S the slope of the IB-smoothed thickness profile 965Aas a function of resonant wavelength as defined by curve 965W, andprovides a first region 1401 from 450 to 600 nm and a second region 1402from 600 to 645 nm, and the reference line λc at 645 nm; and FIG. 14Cgraphs the color response in a*, b* color coordinates of the reflectivepolarizer-absorbing polarizer laminate, with the curves ϕ15, ϕ25, ϕ35,and λ45 having analogous meanings to those of FIG. 10C, and with areference circle C.

For the profile 966: FIG. 15A is a compound graph in which curve 966 isidentical to the profile 966, curve 966A is the IB-smoothed thicknessprofile of curve 966, the curve 966W is the resonant wavelength for theIB-smoothed thickness profile at the specified oblique optical geometry,and reference line λc marks the critical wavelength at 645 nm; FIG. 15Bgraphs as curve 966S the slope of the IB-smoothed thickness profile 966Aas a function of resonant wavelength as defined by curve 966W, andprovides a first region 1501 from 450 to 600 nm and a second region 1502from 600 to 645 nm, and the reference line λc at 645 nm; and FIG. 15Cgraphs the color response in a*, b* color coordinates of the reflectivepolarizer-absorbing polarizer laminate, with the curves ϕ15, ϕ25, ϕ35,and ϕ45 having analogous meanings to those of FIG. 10C, and with areference circle C.

For the profile 967: FIG. 16A is a compound graph in which curve 967 isidentical to the profile 967, curve 967A is the IB-smoothed thicknessprofile of curve 967, the curve 967W is the resonant wavelength for theIB-smoothed thickness profile at the specified oblique optical geometry,and reference line λc marks the critical wavelength at 645 nm; FIG. 16Bgraphs as curve 967S the slope of the IB-smoothed thickness profile 967Aas a function of resonant wavelength as defined by curve 967W, andprovides a first region 1601 from 450 to 600 nm and a second region 1602from 600 to 645 nm, and the reference line λc at 645 nm; and FIG. 16Cgraphs the color response in a*, b* color coordinates of the reflectivepolarizer-absorbing polarizer laminate, with the curves ϕ15, ϕ25, ϕ35,and ϕ45 having analogous meanings to those of FIG. 10C, and with areference circle C.

For the profile 968: FIG. 17A is a compound graph in which curve 968 isidentical to the profile 968, curve 968A is the IB-smoothed thicknessprofile of curve 968, the curve 968W is the resonant wavelength for theIB-smoothed thickness profile at the specified oblique optical geometry,and reference line λc marks the critical wavelength at 645 nm; FIG. 17Bgraphs as curve 968S the slope of the IB-smoothed thickness profile 968Aas a function of resonant wavelength as defined by curve 968W, andprovides a first region 1701 from 450 to 600 nm and a second region 1702from 600 to 645 nm, and the reference line λc at 645 nm; and FIG. 17Cgraphs the color response in a*, b* color coordinates of the reflectivepolarizer-absorbing polarizer laminate, with the curves ϕ15, ϕ25, ϕ35,and ϕ45 having analogous meanings to those of FIG. 10C, and with areference circle C.

The results of the slope ratio for the embodiments of FIG. 9 aresummarized in Table 2, where the “Slope Ratio” refers to the secondaverage of the slope of the IB-smoothed thickness profile (from 600 to645 nm) divided by the first average of the slope of that profile (from450 to 600 nm).

TABLE 2 thickness profile in FIG. 9 Slope Ratio 961 1.00 962 1.27 9631.53 964 1.76 965 1.96 966 2.12 967 2.26 968 2.39

The color response of these embodiments, for a viewer at the highlyoblique critical viewing angle, is best evaluated by inspection of thecolor response curves in FIGS. 10C, 11C, 12C, . . . 17C. In briefsummary: the color trajectories of FIGS. 10C, 11C, 12C, and 13C (for ORUthickness profiles 961, 962, 963, and 964, respectively) are withinacceptable color limits; but the color trajectories of FIGS. 14C, 15C,16C, and 17C (for ORU thickness profiles 965, 966, 967, and 968respectively) are too wide, i.e., they produce an excessive amount ofcolor.

FIG. 18 and its related FIGS. 19A through 26C demonstrate theapplication of these same principles to other related (modeled)embodiments of TOP reflective polarizers and laminates thereof with ahigh contrast absorbing polarizer. Similar to the embodiments of FIG. 9,the TOP reflective polarizer embodiments of FIG. 18 also have exactly152 ORUs, and each ORU has only two microlayers, whose refractiveindices are again as provided above in Table 1. FIG. 18 depicts eightdifferent but related ORU physical thickness profiles 1861, 1862, 1863,1864, 1865, 1866, 1867, and 1868, any of which may be readily employedin a TOP reflective polarizer. The thickness profile 1861 is linear inform, i.e. of constant slope, from the first ORU (#1), whose physicalthickness is about 125 nm, to the last ORU (#152), whose physicalthickness is about 275 nm. The other thickness profiles 1862-1868 areidentical to the thickness profile 1861 from ORU #1 through ORU #125,but then at ORU #125 they undergo a step-change in the slope ofthickness profile from ORU #125 to ORU #152. The step change in slope issmallest for profile 1862 and largest for profile 1868, as shown in thefigure.

Each of these ORU thickness profiles was then analyzed in substantiallythe same way as described above in connection with FIGS. 9 through 17C,which analysis will not be repeated here to avoid needless repetition.

For the ORU thickness profile 1861: FIG. 19A is a compound graph inwhich curve 1861 is identical to the profile 1861, curve 1861A is theIB-smoothed thickness profile of curve 1861, the curve 1861W is theresonant wavelength for the IB-smoothed thickness profile at thespecified oblique optical geometry, and reference line λc marks thecritical wavelength at 645 nm; FIG. 19B graphs as curve 1861S the slopeof the IB-smoothed thickness profile 1861A as a function of resonantwavelength as defined by curve 1861W, and provides a first region 1901from 450 to 600 nm and a second region 1902 from 600 to 645 nm, and thereference line λc at 645 nm; and FIG. 19C graphs the color response ina*, b* color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogousmeanings to those of FIG. 10C, and with a reference circle C.

For the ORU thickness profile 1862: FIG. 20A is a compound graph inwhich curve 1862 is identical to the profile 1862, curve 1862A is theIB-smoothed thickness profile of curve 1862, the curve 1862W is theresonant wavelength for the IB-smoothed thickness profile at thespecified oblique optical geometry, and reference line λc marks thecritical wavelength at 645 nm; FIG. 20B graphs as curve 1862S the slopeof the IB-smoothed thickness profile 1862A as a function of resonantwavelength as defined by curve 1862W, and provides a first region 2001from 450 to 600 nm and a second region 2002 from 600 to 645 nm, and thereference line λc at 645 nm; and FIG. 20C graphs the color response ina*, b* color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogousmeanings to those of FIG. 10C, and with a reference circle C.

For the ORU thickness profile 1863: FIG. 21A is a compound graph inwhich curve 1863 is identical to the profile 1863, curve 1863A is theIB-smoothed thickness profile of curve 1863, the curve 1863W is theresonant wavelength for the IB-smoothed thickness profile at thespecified oblique optical geometry, and reference line λc marks thecritical wavelength at 645 nm; FIG. 21B graphs as curve 1863S the slopeof the IB-smoothed thickness profile 1863A as a function of resonantwavelength as defined by curve 1863W, and provides a first region 2101from 450 to 600 nm and a second region 2102 from 600 to 645 nm, and thereference line λc at 645 nm; and FIG. 21C graphs the color response ina*, b* color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogousmeanings to those of FIG. 10C, and with a reference circle C.

For the ORU thickness profile 1864: FIG. 22A is a compound graph inwhich curve 1864 is identical to the profile 1864, curve 1864A is theIB-smoothed thickness profile of curve 1864, the curve 1864W is theresonant wavelength for the IB-smoothed thickness profile at thespecified oblique optical geometry, and reference line λc marks thecritical wavelength at 645 nm; FIG. 22B graphs as curve 1864S the slopeof the IB-smoothed thickness profile 1864A as a function of resonantwavelength as defined by curve 1864W, and provides a first region 2201from 450 to 600 nm and a second region 2202 from 600 to 645 nm, and thereference line λc at 645 nm; and FIG. 22C graphs the color response ina*, b* color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogousmeanings to those of FIG. 10C, and with a reference circle C.

For the ORU thickness profile 1865: FIG. 23A is a compound graph inwhich curve 1865 is identical to the profile 1865, curve 1865A is theIB-smoothed thickness profile of curve 1865, the curve 1865W is theresonant wavelength for the IB-smoothed thickness profile at thespecified oblique optical geometry, and reference line λc marks thecritical wavelength at 645 nm; FIG. 23B graphs as curve 1865S the slopeof the IB-smoothed thickness profile 1865A as a function of resonantwavelength as defined by curve 1865W, and provides a first region 2301from 450 to 600 nm and a second region 2302 from 600 to 645 nm, and thereference line λc at 645 nm; and FIG. 23C graphs the color response ina*, b* color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogousmeanings to those of FIG. 10C, and with a reference circle C.

For the ORU thickness profile 1866: FIG. 24A is a compound graph inwhich curve 1866 is identical to the profile 1866, curve 1866A is theIB-smoothed thickness profile of curve 1866, the curve 1866W is theresonant wavelength for the IB-smoothed thickness profile at thespecified oblique optical geometry, and reference line λc marks thecritical wavelength at 645 nm; FIG. 24B graphs as curve 1866S the slopeof the IB-smoothed thickness profile 1866A as a function of resonantwavelength as defined by curve 1866W, and provides a first region 2401from 450 to 600 nm and a second region 2402 from 600 to 645 nm, and thereference line λc at 645 nm; and FIG. 24C graphs the color response ina*, b* color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogousmeanings to those of FIG. 10C, and with a reference circle C.

For the ORU thickness profile 1867: FIG. 25A is a compound graph inwhich curve 1867 is identical to the profile 1867, curve 1867A is theIB-smoothed thickness profile of curve 1867, the curve 1867W is theresonant wavelength for the IB-smoothed thickness profile at thespecified oblique optical geometry, and reference line λc marks thecritical wavelength at 645 nm; FIG. 25B graphs as curve 1867S the slopeof the IB-smoothed thickness profile 1867A as a function of resonantwavelength as defined by curve 1867W, and provides a first region 2501from 450 to 600 nm and a second region 2502 from 600 to 645 nm, and thereference line λc at 645 nm; and FIG. 25C graphs the color response ina*, b* color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogousmeanings to those of FIG. 10C, and with a reference circle C.

For the ORU thickness profile 1868: FIG. 26A is a compound graph inwhich curve 1868 is identical to the profile 1868, curve 1868A is theIB-smoothed thickness profile of curve 1868, the curve 1868W is theresonant wavelength for the IB-smoothed thickness profile at thespecified oblique optical geometry, and reference line λc marks thecritical wavelength at 645 nm; FIG. 26B graphs as curve 1868S the slopeof the IB-smoothed thickness profile 1868A as a function of resonantwavelength as defined by curve 1868W, and provides a first region 2601from 450 to 600 nm and a second region 2602 from 600 to 645 nm, and thereference line λc at 645 nm; and FIG. 26C graphs the color response ina*, b* color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogousmeanings to those of FIG. 10C, and with a reference circle C.

The results of the slope ratio for the embodiments of FIG. 18 aresummarized in Table 3, where the “Slope Ratio” has the same meaning asin Table 2 above.

TABLE 3 thickness profile in FIG. 18 Slope Ratio 1861 1.00 1862 1.011863 1.03 1864 1.04 1865 1.06 1866 1.05 1867 1.07 1868 1.08

The color response of these embodiments, for a viewer at the highlyoblique critical viewing angle, is best evaluated by inspection of thecolor response curves in FIGS. 19C, 20C, 21C, . . . 26C. In briefsummary, the color trajectories for all of these embodiments remainwithin acceptable limits.

FIG. 27 and its related FIGS. 28A through 30C demonstrate theapplication of these same principles to still more related (modeled)embodiments of TOP reflective polarizers and laminates thereof with ahigh contrast absorbing polarizer. Similar to the embodiments of FIGS. 9and 18, the TOP reflective polarizer embodiments of FIG. 27 also haveexactly 152 ORUs, and each ORU has only two microlayers, whoserefractive indices are again as provided above in Table 1. FIG. 27depicts three different but related ORU physical thickness profiles2761, 2762, and 2763, any of which may be readily employed in a TOPreflective polarizer. The thickness profile 2762 is linear in form, i.e.of constant slope, from the first ORU (#1), whose physical thickness isabout 108 nm, to the last ORU (#152), whose physical thickness is about255 nm. The other thickness profiles 2761 and 2763 are also linear inform, but related to the thickness profile 2762 by a simple scalingfactor. The profile 2761 is derived by multiplying the profile 2762 by ascaling factor of 95%. The profile 2763 is derived by multiplying theprofile 2762 by a scaling factor of 105%.

Each of these ORU thickness profiles was then analyzed in substantiallythe same way as described above in connection with FIGS. 9 through 26C,which analysis will not be repeated here to avoid needless repetition.

For the ORU thickness profile 2761: FIG. 28A is a compound graph inwhich curve 2761 is identical to the profile 2761, curve 2761A is theIB-smoothed thickness profile of curve 2761, the curve 2761W is theresonant wavelength for the IB-smoothed thickness profile at thespecified oblique optical geometry, and reference line λc marks thecritical wavelength at 645 nm; FIG. 28B graphs as curve 2761S the slopeof the IB-smoothed thickness profile 2761A as a function of resonantwavelength as defined by curve 2761W, and provides a first region 2801from 450 to 600 nm and a second region 2802 from 600 to 645 nm, and thereference line λc at 645 nm; and FIG. 28C graphs the color response ina*, b* color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ15, ϕ25, and ϕ35 having analogous meanings tothose of FIG. 10C, and with a reference circle C.

For the ORU thickness profile 2762: FIG. 29A is a compound graph inwhich curve 2762 is identical to the profile 2762, curve 2762A is theIB-smoothed thickness profile of curve 2762, the curve 2762W is theresonant wavelength for the IB-smoothed thickness profile at thespecified oblique optical geometry, and reference line λc marks thecritical wavelength at 645 nm; FIG. 29B graphs as curve 2762S the slopeof the IB-smoothed thickness profile 2762A as a function of resonantwavelength as defined by curve 2762W, and provides a first region 2901from 450 to 600 nm and a second region 2902 from 600 to 645 nm, and thereference line λc at 645 nm; and FIG. 29C graphs the color response ina*, b* color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ15, ϕ25, and ϕ35 having analogous meanings tothose of FIG. 10C, and with a reference circle C.

For the ORU thickness profile 2763: FIG. 30A is a compound graph inwhich curve 2763 is identical to the profile 2763, curve 2763A is theIB-smoothed thickness profile of curve 2763, the curve 2763W is theresonant wavelength for the IB-smoothed thickness profile at thespecified oblique optical geometry, and reference line λc marks thecritical wavelength at 645 nm; FIG. 30B graphs as curve 2763S the slopeof the IB-smoothed thickness profile 2763A as a function of resonantwavelength as defined by curve 2763W, and provides a first region 3001from 450 to 600 nm and a second region 3002 from 600 to 645 nm, and thereference line λc at 645 nm; and FIG. 30C graphs the color response ina*, b* color coordinates of the reflective polarizer-absorbing polarizerlaminate, with the curves ϕ5, ϕ25, ϕ35, and ϕ45 having analogousmeanings to those of FIG. 10C, and with a reference circle C.

The results of the slope ratio for the embodiments of FIG. 27 aresummarized in Table 4, where the “Slope Ratio” has the same meaning asin Tables 2 and 3 above.

TABLE 4 thickness profile in FIG. 27 Slope Ratio 2761 undefined 27620.84 2763 1.0

The color response of these embodiments, for a viewer at the highlyoblique critical viewing angle, is best evaluated by inspection of thecolor response curves in FIGS. 28C, 29C, and 30C. In brief summary: thecolor trajectory of FIG. 28C (for ORU thickness profile 2761) produces asignificant unacceptable red color; but the color trajectories of FIGS.29C and 30C (for ORU thickness profiles 2762, 2763 respectively) remainwithin acceptable limits.

EXAMPLE AND COMPARATIVE EXAMPLES

Some polymer-based TOP reflective polarizer films were fabricated oncontinuous film lines by procedures that included polymer coextrusionthrough a feedblock, quenching, and tentering. Such films, and in somecases laminates of such films with an aligned high contrast absorbingpolarizer, were also tested. Some of the testing involved measuring thethickness profile of the microlayer stack with an AFM device. Othertesting involved observing a piece of the polarizer film, or laminatethereof, on a light table at highly oblique optical geometries.

In a first case, referred to herein as the “Example”, a TOP reflectivepolarizer film was made in accordance with known multilayer optical filmfabrication techniques in which the biaxially birefringent microlayersof the microlayer packet comprised LmPEN as described above, and theisotropic microlayers of the microlayer packet comprised an amorphousblend of PETg GN071 (Eastman Chemicals, Knoxville, Tenn.) and LmPEN atthe weight fraction of 58% and 42%, respectively. The refractive indicesof these polymers were similar to those in Table 1. An axial rod heater,as described for example in U.S. Pat. No. 6,783,349 (Neavin et al.), wasemployed in the feedblock, and the temperature profile along the axialrod heater was used to provide some control over the polymerflowstreams, and hence also over the ORU thickness profile in thefinished TOP reflective polarizer film. The microlayer packet in suchfilm contained 152 ORUs, each ORU having one biaxially birefringentmicrolayer and one isotropic microlayer. The physical thickness of theTOP reflective polarizer was about 31 microns. The Example TOPreflective polarizer provided a block axis and a pass axis, and a wideband reflectivity for normally incident light polarized along the blockaxis, the reflectivity for such normally incident polarized light beinggreater than 90% from 430 nm to 650 nm.

Layer thicknesses in the finished film were measured with an AFM device.The raw AFM thickness output was conditioned using an averagingtechnique to filter out noise and obtain more accurate thickness values.The resulting measured thicknesses of the ORUs are plotted as ORUphysical thickness profile 3161 in FIG. 31. Inspection of the graphreveals the microlayer packet had a first end, at ORU #1, and a secondend, at ORU #151, and ORUs proximate the second end had an averagephysical thickness greater than that of ORUs proximate the first end.

An analysis was done on the profile 3161 in substantially the samemanner as described in connection with the other embodiments above. Inthat regard: FIG. 32A is a compound graph in which curve 3161 isidentical to the profile 3161, curve 3161A is the IB-smoothed thicknessprofile of curve 3161, the curve 3161W is the resonant wavelength forthe IB-smoothed thickness profile at the specified oblique opticalgeometry, and reference line λc marks the critical wavelength at 645 nm;FIG. 32B graphs as curve 3161S the slope of the IB-smoothed thicknessprofile 3161A as a function of resonant wavelength as defined by curve3161W, and provides a first region 3201 from 450 to 600 nm and a secondregion 3202 from 600 to 645 nm, and the reference line λc at 645 nm; andFIG. 32C graphs the color response in a*, b* color coordinates of a(modeled) laminate of a reflective polarizer (as characterized by theprofile 3161) with an aligned, high contrast absorbing polarizer(contrast=1000), and with the TOP reflective polarizer oriented so thatthe end of the packet with generally thicker ORUs is closer to theabsorbing polarizer than the end of the packet with generally thinnerORUs, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogous meaningsto those of FIG. 10C, and with a reference circle C.

The slope ratio for the Example, calculated in the same fashion as theSlope Ratio in Tables 2, 3, and 4 above, was equal to 1.34. This value,being less than 1.8, is consistent with the observation that the colorresponse curves in FIG. 32C remain substantially within acceptablelimits, exhibiting acceptable color uniformity behavior and remainingsubstantially white in color within the critical high angle viewingrange.

A physical sample of the Example TOP reflective polarizer film was alsovisually tested and observed on a light table. Clear glass was laminatedto a San Ritz 5618 absorbing polarizer (San Ritz, Tokyo, Japan) usingthe adhesive that is a component of the San Ritz 5618 polarizer. The TOPreflective polarizer film was then laminated using an optically clearadhesive (OCA 8171 from 3M, Saint Paul, Minn.) to the absorbingpolarizer. In order to view the color characteristics of this laminateat a variety of angles, a diffuse light table was utilized using thetype of white LEDs commonly found in LCD backlights. The laminate wasplaced on the light table with the reflective polarizer side down. Thelaminate was then viewed over the entire hemisphere. The example filmshowed no objectionable colors or color uniformity even when viewed atthe most severe angles (θ=80 degrees and ϕ=15, 25, 35, and 45 degrees).The example laminate was compared to another laminate made using thereflective polarizer contained within a commercialized on-glassreflective polarizer (APCF from Nitto Denko, Tokyo, Japan). The laminatemade using the APCF reflective polarizer was constructed of the samelayers as the Example laminate: glass/adhesive/absorbingpolarizer/adhesive/reflecting polarizer, with the absorbing polarizerbeing the same as that used in the Example laminate (but where thereflective polarizer was the APCF reflective polarizer). When viewed atthe angles described above, both laminates had very small color shiftswith angle and very small spatial color uniformity that were judged tobe similar in intensity.

In another case, referred to herein as “Comparative Example 1”, a TOPreflective polarizer film was made in accordance with known multilayeroptical film fabrication techniques in which the biaxially birefringentmicrolayers of the microlayer packet comprised LmPEN as described above,and the isotropic microlayers of the microlayer packet comprised anamorphous blend of PETg GN071 (Eastman Chemicals, Knoxville, Tenn.) andLmPEN at the weight fraction of 58% and 42%, respectively. Therefractive indices of these polymers were similar to those in Table 1.The microlayer packet in such film contained 152 ORUs, each ORU havingone biaxially birefringent microlayer and one isotropic microlayer. Thephysical thickness of the TOP reflective polarizer was about 31 microns.

Layer thicknesses in the finished film were measured with an AFM device.The raw AFM thickness output was conditioned using an averagingtechnique to filter out noise and obtain more accurate thickness values.The resulting measured thicknesses of the ORUs are plotted as ORUphysical thickness profile 3361 in FIG. 33. Inspection of the graphreveals the microlayer packet had a first end, at ORU #1, and a secondend, at ORU #151, and ORUs proximate the second end had an averagephysical thickness greater than that of ORUs proximate the first end.

An analysis was done on the profile 3361 in substantially the samemanner as described in connection with the other embodiments above. Inthat regard: FIG. 34A is a compound graph in which curve 3361 isidentical to the profile 3361, curve 3361A is the IB-smoothed thicknessprofile of curve 3361, the curve 3361W is the resonant wavelength forthe IB-smoothed thickness profile at the specified oblique opticalgeometry, and reference line λc marks the critical wavelength at 645 nm;FIG. 34B graphs as curve 3361S the slope of the IB-smoothed thicknessprofile 3361A as a function of resonant wavelength as defined by curve3361W, and provides a first region 3401 from 450 to 600 nm and a secondregion 3402 from 600 to 645 nm, and the reference line λc at 645 nm; andFIG. 34C graphs the color response in a*, b* color coordinates of a(modeled) laminate of a reflective polarizer (as characterized by theprofile 3361) with an aligned, high contrast absorbing polarizer(contrast=1000), and with the TOP reflective polarizer oriented so thatthe end of the packet with generally thicker ORUs is closer to theabsorbing polarizer than the end of the packet with generally thinnerORUs, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogous meaningsto those of FIG. 10C, and with a reference circle C.

The slope ratio for the Comparative Example 1, calculated in the samefashion as the Slope Ratio in Tables 2, 3, and 4 above, was equal to2.62. This value, being greater than 1.8, is consistent with theobservation that the color response curves in FIG. 34C exhibitunacceptable color uniformity behavior, creating a white-to-yelloweffect, within the critical high angle viewing range.

A physical sample of the Comparative Example 1 TOP reflective polarizerfilm was also visually tested and observed on a light table. Clear glasswas laminated to a San Ritz 5618 absorbing polarizer (San Ritz, Tokyo,Japan) using the adhesive that is a component of the San Ritz 5618polarizer. The Comparative Example 1 film was then laminated using anoptically clear adhesive (OCA 8171 from 3M, Saint Paul, Minn.) to theabsorbing polarizer. In order to view the color characteristics of thislaminate at a variety of angles, a diffuse light table was utilizedusing the type of white LEDs commonly found in LCD backlights. Thelaminate was placed on the light table with the reflective polarizerside down. The laminate was then viewed over the entire hemisphere. TheComparative Example 1 laminate showed objectionable colors, at leastwhen viewed at some of the most severe angles (θ=80 degrees and ϕ=15,25, 35, and 45 degrees). When the Comparative Example 1 laminate wascompared to the laminate made using the APCF reflective polarizer(described above), it was judged that the Comparative Example 1 laminatehad much more severe color shift with angle and spatial color variation,and was too severe to be considered for use in a high fidelity display.

In still another case, referred to herein as “Comparative Example 2”, aTOP reflective polarizer film was made in accordance with knownmultilayer optical film fabrication techniques in which the biaxiallybirefringent microlayers of the microlayer packet comprised LmPEN, andthe isotropic microlayers of the microlayer packet comprised anamorphous blend of PETg GN071 (Eastman Chemicals, Knoxville, Tenn.) andLmPEN at the weight fraction of 58% and 42%, respectively. ThisComparative Example 2 TOP reflective polarizer film is substantiallysimilar to the reflective polarizer film described in U.S. Pat. No.7,791,687 (Weber et al.), at column 10, lines 9-46 and in FIG. 9thereof. The refractive indices of the birefringent polymer werenx=1.820, ny=1.575, and nz=1.560, and the refractive index of theisotropic polymer was 1.595. The microlayer packet in such filmcontained 138 ORUs, each ORU having one biaxially birefringentmicrolayer and one isotropic microlayer. The physical thickness of theTOP reflective polarizer was 31 microns.

Layer thicknesses in the finished film were measured with an AFM device.The raw AFM thickness output was conditioned using an averagingtechnique to filter out noise and obtain more accurate thickness values.The resulting measured thicknesses of the ORUs are plotted as ORUphysical thickness profile 3561 in FIG. 35, which is substantiallysimilar to FIG. 9 of the '687 Weber et al. patent. Inspection of thegraph reveals the microlayer packet had a first end, at ORU #1, and asecond end, at ORU #137, and ORUs proximate the second end had anaverage physical thickness greater than that of ORUs proximate the firstend.

An analysis was done on the profile 3561 in substantially the samemanner as described in connection with the other embodiments above. Inthat regard: FIG. 36A is a compound graph in which curve 3561 isidentical to the profile 3561, curve 3561A is the IB-smoothed thicknessprofile of curve 3561, the curve 3561W is the resonant wavelength forthe IB-smoothed thickness profile at the specified oblique opticalgeometry, and reference line λc marks the critical wavelength at 645 nm;FIG. 36B graphs as curve 3561S the slope of the IB-smoothed thicknessprofile 3561A as a function of resonant wavelength as defined by curve3561W, and provides a first region 3601 from 450 to 600 nm and a secondregion 3602 from 600 to 645 nm, and the reference line λc at 645 nm; andFIG. 36C graphs the color response in a*, b* color coordinates of a(modeled) laminate of a reflective polarizer (as characterized by theprofile 3561) with an aligned, high contrast absorbing polarizer(contrast=1000), and with the TOP reflective polarizer oriented so thatthe end of the packet with generally thicker ORUs is closer to theabsorbing polarizer than the end of the packet with generally thinnerORUs, with the curves ϕ15, ϕ25, ϕ35, and ϕ45 having analogous meaningsto those of FIG. 10C, and with a reference circle C.

The slope ratio for the Comparative Example 2, calculated in the samefashion as the Slope Ratio in Tables 2, 3, and 4 above, was equal to1.91. This value, being greater than 1.8, is consistent with theobservation that the color response curves in FIG. 36C exhibitunacceptable color uniformity behavior, creating a white-to-red effect,within the critical high angle viewing range.

A physical sample of the Comparative Example 2 TOP reflective polarizerfilm was also visually tested and observed on a light table. Clear glasswas laminated to a San Ritz 5618 absorbing polarizer (San Ritz, Tokyo,Japan) using the adhesive that is a component of the San Ritz 5618polarizer. The Comparative Example 2 film was then laminated using anoptically clear adhesive (OCA 8171 from 3M, Saint Paul, Minn.) to theabsorbing polarizer. In order to view the color characteristics of thislaminate at a variety of angles, a diffuse light table was utilizedusing the type of white LEDs commonly found in LCD backlights. Thelaminate was placed on the light table with the reflective polarizerside down. The laminate was then viewed over the entire hemisphere. TheComparative Example 2 laminate showed objectionable colors, at leastwhen viewed at some of the most severe angles (θ=80 degrees and ϕ=15,25, 35, and 45 degrees). When the Comparative Example 2 laminate wascompared to the laminate made using the APCF reflective polarizer(described above), it was judged that the Comparative Example 2 laminatehad much more severe color shift with angle and spatial color variation,and was too severe to be considered for use in a high fidelity display.

We also wish to address the issue of how the reflective polarizer shouldbe oriented relative to the absorbing polarizer in the laminate toachieve the lowest levels of unwanted visible color at high obliqueangles in the white state of the display (in additional to low color atnormal incidence and intermediate oblique angles). We state above thatsuch unwanted visible color can be substantially reduced to acceptablelevels by orienting the TOP polarizer such that the thick microlayer endof the microlayer packet (the side of the TOP polarizer with the thickerORUs, given that there is a gradient along the thickness axis of thepolarizer from predominantly thinner to predominantly thicker ORUs) isadjacent to, and faces, the absorbing polarizer (and the front of thedisplay), and the thinner microlayer end faces the back of the displayand away from the absorbing polarizer. Referring to this as thethick-layers-up orientation, and the opposite orientation as thethin-layers-up orientation, the Example laminate, the ComparativeExample 1 laminate, and the Comparative Example 2 laminate all used thethick-layers-up orientation. Each of those three laminates was howeveralso investigated in a reconfigured thin-layers-up orientation using thediffuse light table. In each case, the color performance at the mostsevere observation geometry (for example, θ=80 degrees and ϕ=15, 25, 35,and 45 degrees) was inferior to the color performance in thethick-layers-up orientation.

Following is a non-comprehensive list of some items discussed anddescribed in the present disclosure.

Item 1 is a laminate, comprising:

-   -   a reflective polarizer having only one packet of microlayers        that reflects and transmits light by optical interference, the        packet of microlayers configured to define a first pass axis        (y), a first block axis (x), and a first thickness axis (z)        perpendicular to the first pass axis and the first block axis,        the first block axis and the first thickness axis forming an x-z        plane, the packet of microlayers comprising alternating first        and second microlayers, the first microlayers being biaxially        birefringent; and    -   an absorbing polarizer having a second pass axis and a second        block axis, the absorbing polarizer attached to the reflective        polarizer with no air gap therebetween and such that the first        and second pass axes are substantially aligned, the absorbing        polarizer having a contrast ratio of at least 1000;    -   wherein adjacent pairs of the first and second microlayers form        optical repeat units (ORUs) along the packet of microlayers, the        ORUs defining a physical thickness profile having a gradient        that provides a wide band reflectivity for normally incident        light polarized along the first block axis, the ORUs having        respective resonant wavelengths as a function of the physical        thickness profile and optical geometry;    -   wherein the ORUs include a first ORU and a last ORU defining        opposite ends of the packet, the last ORU being closer than the        first ORU to the absorbing polarizer, and wherein the physical        thickness profile is such that ORUs proximate the last ORU have        an average physical thickness greater than that of ORUs        proximate the first ORU;    -   wherein an intrinsic-bandwidth based boxcar average of the        physical thickness profile yields an IB-smoothed thickness        profile, the IB-smoothed thickness profile being defined at each        of the ORUs;    -   wherein the ORUs further include:        -   an ORU(450) having a resonant wavelength, for the            IB-smoothed thickness profile, of at least 450 nm for an            oblique optical geometry in which p-polarized light is            incident in the x-z plane at a polar angle (θ) of 80            degrees, all of the ORUs disposed on a side of the ORU(450)            that includes the first ORU having resonant wavelengths less            than 450 nm for the IB-smoothed thickness profile at the            oblique optical geometry;        -   an ORU(600) having a resonant wavelength, for the            IB-smoothed thickness profile, of at least 600 nm for the            oblique optical geometry, all of the ORUs disposed on a side            of the ORU(600) that includes the first ORU having resonant            wavelengths less than 600 nm for the IB-smoothed thickness            profile at the oblique optical geometry; and        -   an ORU(645) which optionally may be the same as the last            ORU, the ORU(645) having a resonant wavelength, for the            IB-smoothed thickness profile, of at least 645 nm for the            oblique optical geometry, all of the ORUs disposed on a side            of the ORU(645) that includes the first ORU having resonant            wavelengths less than 645 nm for the IB-smoothed thickness            profile at the oblique optical geometry; and    -   wherein the IB-smoothed thickness profile has a first average        slope over a range from ORU(450) to ORU(600), and a second        average slope over a range from ORU(600) to ORU(645), and a        ratio of the second average slope to the first average slope is        no more than 1.8.

Item 2 is the laminate of item 1, wherein the IB-smoothed thicknessprofile, as evaluated at any given ORU, encompasses substantially onlythose ORUs that coherently contribute to a reflectivity of the packet ata resonant wavelength of the given ORU.

Item 3 is the laminate of item 1, wherein the IB-smoothed thicknessprofile, as evaluated at any given ORU, encompasses a predeterminednumber of the ORUs that are nearest neighbors on each side of the givenORU.

Item 4 is the laminate of item 3, wherein the predetermined number is nomore than 20.

Item 5 is the laminate of item 3, wherein the predetermined number is atleast 5.

Item 6 is the laminate of item 3, wherein the predetermined number is10.

Item 7 is the laminate of any preceding item, wherein the resonantwavelength of the ORU(450), for the IB-smoothed thickness profile, isless than 455 nm, and the resonant wavelength of the ORU(600), for theIB-smoothed thickness profile, is less than 605 nm, and the resonantwavelength of the ORU(645), for the IB-smoothed thickness profile, isless than 650 nm.

Item 8 is the laminate of any preceding item, wherein the secondmicrolayers are substantially isotropic.

Item 9 is the laminate of any preceding item, wherein first and secondmicrolayers respectively comprise different first and second polymermaterials.

Item 10 is the laminate of any preceding item, wherein the reflectivepolarizer has a physical thickness of less than 50 microns.

Item 11 is the laminate of item 10, wherein the physical thickness ofthe reflective polarizer is in a range from 20 to 40 microns.

Item 12 is the laminate of any preceding item, wherein the laminateconsists essentially of the reflective polarizer, the absorbingpolarizer, and an adhesive layer that bonds the reflective polarizer tothe absorbing polarizer.

Item 13 is the laminate of any preceding item, wherein the packet ofmicrolayers provides the reflective polarizer with a normal incidencetransmission, on average over a wavelength range from 400-700 nm, of atleast 80% for a pass state polarization and less than 15% for a blockstate polarization.

Item 14 is a reflective polarizer having only one packet of microlayersthat reflects and transmits light by optical interference, the packet ofmicrolayers configured to define a pass axis (y), a block axis (x), anda thickness axis (z) perpendicular to the pass axis and the block axis,the block axis and the thickness axis forming an x-z plane, the packetof microlayers comprising alternating first and second microlayers, thefirst microlayers being biaxially birefringent;

-   -   wherein adjacent pairs of the first and second microlayers form        optical repeat units (ORUs) along the packet of microlayers, the        ORUs defining a physical thickness profile having a gradient        that provides a wide band reflectivity for normally incident        light polarized along the block axis, the ORUs having respective        resonant wavelengths as a function of the physical thickness        profile and optical geometry;    -   wherein the ORUs include a first ORU and a last ORU defining        opposite ends of the packet, and wherein the physical thickness        profile is such that ORUs proximate the last ORU have an average        physical thickness greater than that of ORUs proximate the first        ORU;    -   wherein an intrinsic-bandwidth based boxcar average of the        physical thickness profile yields an IB-smoothed thickness        profile, the IB-smoothed thickness profile being defined at each        of the ORUs;    -   wherein the ORUs further include:        -   an ORU(450) having a resonant wavelength, for the            IB-smoothed thickness profile, of at least 450 nm for an            oblique optical geometry in which p-polarized light is            incident in the x-z plane at a polar angle (θ) of 80            degrees, all of the ORUs disposed on a side of the ORU(450)            that includes the first ORU having resonant wavelengths less            than 450 nm for the IB-smoothed thickness profile at the            oblique optical geometry;        -   an ORU(600) having a resonant wavelength, for the            IB-smoothed thickness profile, of at least 600 nm for the            oblique optical geometry, all of the ORUs disposed on a side            of the ORU(600) that includes the first ORU having resonant            wavelengths less than 600 nm for the IB-smoothed thickness            profile at the oblique optical geometry; and        -   an ORU(645) which optionally may be the same as the last            ORU, the ORU(645) having a resonant wavelength, for the            IB-smoothed thickness profile, of at least 645 nm for the            oblique optical geometry, all of the ORUs disposed on a side            of the ORU(645) that includes the first ORU having resonant            wavelengths less than 645 nm for the IB-smoothed thickness            profile at the oblique optical geometry; and    -   wherein the IB-smoothed thickness profile has a first average        slope over a range from ORU(450) to ORU(600), and a second        average slope over a range from ORU(600) to ORU(645), and a        ratio of the second average slope to the first average slope is        no more than 1.8.

Item 15 is the polarizer of item 14, wherein the IB-smoothed thicknessprofile, as evaluated at any given ORU, encompasses substantially onlythose ORUs that coherently contribute to a reflectivity of the packet ata resonant wavelength of the given ORU.

Item 16 is the polarizer of item 14, wherein the IB-smoothed thicknessprofile, as evaluated at any given ORU, encompasses a predeterminednumber of the ORUs that are nearest neighbors on each side of the givenORU.

Item 17 is the polarizer of item 16, wherein the predetermined number isno more than 20.

Item 18 is the polarizer of any of items 14-17, wherein the resonantwavelength of the ORU(450), for the IB-smoothed thickness profile, isless than 455 nm, and the resonant wavelength of the ORU(600), for theIB-smoothed thickness profile, is less than 605 nm, and the resonantwavelength of the ORU(645), for the IB-smoothed thickness profile, isless than 650 nm.

Item 19 is the polarizer of any of items 14-18, wherein the secondmicrolayers are substantially isotropic.

Item 20 is a laminate, comprising:

-   -   the reflective polarizer of item 14, where the pass axis is a        first pass axis and the block axis is a first block axis; and    -   an absorbing polarizer having a second pass axis and a second        block axis, the absorbing polarizer attached to the reflective        polarizer with no air gap therebetween and such that the first        and second pass axes are substantially aligned, the absorbing        polarizer having a contrast ratio of at least 1000;    -   wherein the last ORU is closer than the first ORU to the        absorbing polarizer.

Unless otherwise indicated, all numbers expressing quantities,measurement of properties, and so forth used in the specification andclaims are to be understood as being modified by the term “about”.Accordingly, unless indicated to the contrary, the numerical parametersset forth in the specification and claims are approximations that canvary depending on the desired properties sought to be obtained by thoseskilled in the art utilizing the teachings of the present application.Not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof the invention are approximations, to the extent any numerical valuesare set forth in specific examples described herein, they are reportedas precisely as reasonably possible. Any numerical value, however, maywell contain errors associated with testing or measurement limitations.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the spirit and scopeof this invention, and it should be understood that this invention isnot limited to the illustrative embodiments set forth herein. The readershould assume that features of one disclosed embodiment can also beapplied to all other disclosed embodiments unless otherwise indicated.It should also be understood that all U.S. patents, patent applicationpublications, and other patent and non-patent documents referred toherein are incorporated by reference, to the extent they do notcontradict the foregoing disclosure.

The invention claimed is:
 1. A laminate comprising an absorbingpolarizer having a contrast ratio of at least 1000 and disposed on areflective polarizer with no air gap therebetween, the absorbing andreflective polarizers having substantially aligned pass axes, thereflective polarizer comprising a plurality of alternating first andsecond microlayers, adjacent pairs of the first and second microlayersforming optical repeat units (ORUs) defining a physical thicknessprofile, the ORUs having respective resonant wavelengths as a functionof the physical thickness profile, the ORUs comprising ORU(450),ORU(600) and ORU(650) having respective resonant frequencies of 450, 600and 650 nm, the ORU(650) closer than ORU(450) to the absorbingpolarizer, the physical thickness profile having a first average slopeover a range from ORU(450) to ORU(600), and a second average slope overa range from ORU(600) to ORU(645), a ratio of the second average slopeto the first average slope no more than 1.8.
 2. The laminate of claim 1,wherein the first microlayers are birefringent and the secondmicrolayers are substantially isotropic.
 3. The laminate of claim 1,wherein first and second microlayers comprise different polymermaterials.
 4. The laminate of claim 1, wherein the reflective polarizerhas a physical thickness of less than 50 microns.
 5. The laminate ofclaim 1, wherein the reflective polarizer has a physical thickness in arange from 20 to 40 microns.
 6. The laminate of claim 1 furthercomprising an adhesive layer bonding the reflective polarizer to theabsorbing polarizer.
 7. The laminate of claim 1, wherein for normallyincident light over a wavelength range from 400 nm to 700 nm, thereflective polarizer has an average transmission of at least 80% for thepass polarization state, and an average transmission of less than 15%for a block polarization state orthogonal to the pass polarizationstate.